Meal replacement shakes are growing more and more popular among certain types of people all around the world. There are, however, different kinds of these shakes, and they have different levels of nutrients, different instructions, and different restrictions. So here's the question: how long should you use meal replacement shakes?
Types of Shakes
Before we dig into recommendations and data, first we should talk about the different kinds of meal replacement shakes you might find out there. There are quite a few of them, so we have a bit to go through here.
First up, you have the old-school meal replacement shake, something like SlimFast. In fact, SlimFast was the original meal replacement shake for the modern era. SlimFast shakes are generally formulated to be most of the nutrients you need, but not everything. You're encouraged to replace one meal with them, or maybe two, but you should also eat a regular meal at some point each day, or several smaller, low-calorie snacks.
These shakes are meant primarily to fill you up with enough protein and fiber to make you feel full for a while while giving you fewer calories than the meal you would normally be eating. This makes it easier to maintain the caloric deficit necessary to lose weight. 
The second kind of meal replacement shake is the cleanse shake. Some of these come pre-formulated and can be mixed with water to make a shake, while others are simple recipes you put together yourself using fruits, vegetables, supplements, and powders.
Cleanse shakes are meant to be taken once or twice a day, depending on the plan you're using and the size of the shake. They're also meant to replace everything you consume that day, except for water, of course. They are close to (but not entirely) nutritionally complete. They work similarly to intermittent fasting or to a crash diet; they maintain a high caloric deficit, so you lose weight, and they help purge your system of things like sugar and processed foods and help you get over the cravings for them, so you can return to normal eating with fewer cravings and better habits.
The third kind of meal replacement shake is the Soylent style shake. These shakes are engineered to be full meal replacements and are basically intended to be used indefinitely as a replacement for eating.
You see this kind of shake in use by people who simply don't find pleasure in eating and want to get past the chore as quickly as possible or in the type of Silicon Valley entrepreneur who spends 30 hours a day working and doesn't have time for a leisurely meal. They're meant to be fast, nutritionally complete, and exclusive; you eat them instead of other foods.
Now let's start looking at how long you can use them.
The Lower Bound
One way of interpreting the question is not "how long is it safe to use them", but rather "how long does it take for them to work?" if you're wondering how long it takes for a meal replacement shake to kick in.
The simple answer to this is "as soon as you start them." You're not going to drink a SlimFast and lose a pound on the first day, though.
If your goal with a meal replacement shake is to lose weight (which it is for many of you), you need to burn roughly 3,500 calories to lose 1 pound. Remember that your body needs a certain amount of calories to operate normally, which is around 2,000 for the average woman, and 2,500 per day for the average man. Eating exactly that amount of calories each day puts you at maintenance level; you neither gain nor lose weight. Eating more means you gain, and eating less means you lose.
To lose weight, you need to burn more calories and take in fewer calories. How you do it is up to you. Fat burning supplements help you burn more, exercise directly burns more, diets help you take in less, and meal replacements help you take in less.
A full meal replacement plan like SlimFast, where you're primarily eating meal replacement shakes and supplementary meals throughout the day, will help you lose 1-2 pounds per week. How much specifically depends on your exercise levels and how many calories you were eating that you have now replaced.
That's for weight loss. What if your goal with a meal replacement is to simply not have to eat? If that's the case, you see benefits immediately. There are some drawbacks – which we'll get into later under the risks section – but if all you want is to save time, you do that the moment you mix up a shake instead of cooking and eating a meal.
Cleanse Shakes
First, let's start with the cleanse shakes. Cleanse shakes are not meant to be nutritionally complete, and while some of them are, they're not designed to be consumed for long periods of time. The primary reason cleanse shakes work is that they fail to provide everything your body needs, which forces your body to draw from its own stores, which helps dramatically shed weight quickly.
As such, cleanse shakes are not meant to be taken for more than a few days at a time. Some shakes, if they are closer to nutritionally complete, can be taken for a week at a time, but more than that and you run the risk of complications. You don't want to end up in the hospital or suffering from some other kind of ailment because you wanted to cleanse, after all.
Weight Loss Shakes
Weight loss shakes like the SlimFast we've mentioned so many times already are meant to be part of a meal or to replace one meal out of your normal meal plan. For example, you might have a shake for breakfast rather than cereal or pancakes or eggs but still eat a normal lunch and dinner. 
If you're going for a full meal replacement plan, you might have shakes as the primary part of each meal, and sides of vegetables, or small non-shake "snacks" throughout the day of 100-200 calories each. This is important because it gives you some solid food in your diet. We'll discuss why that's important in the risks section.
These kinds of meal replacement plans can be used for weeks at a time, and sometimes longer. If you're replacing one meal a day but eating normally otherwise, you can keep it up indefinitely; there's no limit to how many days in a row you can do so. If you're opting for a more total meal replacement plan, you might want to stick with it for a month or so, but then take a break.
Meal Replacement Shakes
Soylent, Huel, Ambronite, and other total meal replacement shakes are intended to be fully nutritionally complete and to provide enough calories to live on. The goal is to have an actual, real meal replacement, not a supplement. Technically, you can use these indefinitely.
In reality, there are some challenges to this plan, which are basically the same as the challenges you'll face with weight loss shakes as well. We'll dig into it more momentarily, but the biggest is simply that our bodies are not designed for an all-liquid diet. Solid foods provide an important role in our bodies, so even if you're getting the nutrients from the shakes, you may still experience negative side effects.
Risks of Meal Replacement Shakes
There are a handful of different risks you want to be aware of with your meal replacement shakes.
First of all, one risk is unique to juice cleanses: nutritional deficiencies. Cleanses are a stopgap measure, since as are meant to be short-term and sort of like a halfway-point between eating normally and fasting. They help you survive, but they don't help you live. If you rely on them for too long, you'll likely end up with vitamin or mineral deficiencies, or malnutrition of some form or another.
The primary risk of more total meal replacement plans, like dedicated use of weight loss shakes or total use of Soylent and other meal replacements, is that there is no solid food component.
The human body, particularly the digestive system, is designed to take solid foods and convert them into nutritional liquids, extract the nutrition out of them, and excrete the rest. Our teeth and saliva start the process by masticating and breaking down the food. Our stomachs continue to break it down into a slurry that is funneled into the intestines. The intestines let it continue to dissolve while extracting nutrients and condensing the rest.
So what happens if you start with a liquid diet? Well, your body recognizes that it doesn't need to linger in the stomach, and it passes through you pretty quickly. This can range from less constipation to more frequent and more urgent diarrhea.
In part, this is due to your adjustment to a new diet and can happen with other diets like Keto as well. In part, though, it's due to your body not being used to liquid diets.
Another risk is that, while meal replacements like Soylent claim to be nutritionally complete, that might not actually be true. This is not solely due to the assumptions used to develop the meal replacement, but because we're still learning and understanding what nutrition even means, even today. It's difficult to know for sure what is good and what is bad for you, and many things that are "bad" can have good aspects, and vice versa. Alcohol is bad, but a glass of wine a day can have positive effects. Sugar is bad, but fruit – which is packed with fructose – is good. See what we mean?
The all-liquid diet may also run the risk of making you want to eat more. Even though meal replacements tend to be packed with fiber to slow down the digestive process and satiate the appetite, but it's still not as effective as just eating a serving of vegetables.
Then there's the teeth issue. Juice cleanses tend to, well, be based in juices, and juices are acidic. These liquid diets and erode your teeth more effectively than soft drinks. There are also rumors that an extended all-liquid diet could make your teeth loosen and even fall out, though we haven't heard of any actual cases of that happening where there weren't confounding factors like scurvy or malnutrition.
Meal Replacements are Generally Safe
Despite all of the risks above, meal replacements are generally safe so long as you follow the directions. If a meal replacement says to use it for one meal a day, don't replace all three meals with it. If it says to use it for 3-7 days, don't use it for two weeks. If it says you can use it every day for all of your food needs, well, you can do so, but be careful.
We recommend consulting with a doctor or dietician to make sure that you're getting all of the nutrients you need from your diet. After a week or two of a meal replacement diet, get tested, and make sure you're not starting to suffer from a vitamin or nutrition deficiency. If you are, consider what you need to do to adjust; either eating some solid healthy food to fill the gap with some nutrients, or add a supplement to your routine.
You shouldn't be scared of trying a meal replacement because of the potential risks. There are risks to every diet, every weight loss supplement, and every exercise. You just need to know what you're doing, pay attention to your body, and listen to the feedback it gives you. Drinking a meal replacement shake for every meal for months on end probably isn't good for you, but there's no harm in enjoying them every now and again.
