In your cleansing routine, how often do you clean different parts of your body?
Cleaning your face? Easy; do that once or twice a day. Twice, once in the morning and once before bed, is ideal to avoid overnight breakouts and blemishes.
The rest of your body? Well, best practice is to shower or otherwise bathe once a day. Keeping the pits and other stanky places clean is a no brainer. A good bodywash and a scrub every day keeps your skin healthy and flush, and keeps your friends close.
What about your hair?
The easy answer is just to reach for the shampoo as part of your daily routine. Finish scrubbing down and treat your hair, or vice versa; get the hair done and work from the top down.
The actual answer depends on a handful of different factors. There's no one easy answer. Take a look at these questions and see how they shake out for you.
Is Your Hair Naturally Oily?
The anatomy of hair is more complicated than it might seem. Hair pokes out from the surface of the skin from a follicle, but there's more to it than just a hole that grows hair. The follicle includes sebaceous glands, which produce sebum. Sebum is a sort of oily, waxy stuff that helps moisturize and protect your skin, scalp, and hair. It's a fantastically complex substance, made up of sugars, fatty acids, waxes, oils, and other chemicals all derived from what you eat and what your body processes. 
Sebum is protective and restorative, but it's also responsible for the oily or greasy look and feel your hair – and skin, for that matter – will have if left alone long enough. On its own, it isn't bad, but since it's oily, it also tends to accumulate dust and dirt from the air.
There are three factors at play here. First of all, different people produce different amounts of sebum each day. Some people are just naturally oilier than other people, and there's not a lot of rhyme or reason to it.
Second, your diet plays a role. If you have a generally healthy diet, you're typically going to produce more sebum. Your body naturally prioritizes different aspects of nutrition throughout your life. Obviously, critical internal organs top the list, and less critical organs – like skin, nails, and peripheral blood vessels – come in second. Other processes, like sebum production, are lower on the priority list. Your body isn't going to waste nutrients on bodily oils when it needs them to keep your heart pumping, after all.
Third, there's some evidence that suggests the more frequently you wash your hair, the more your body fights to restore the sebum you're washing away. In other words, the more you wash your hair, the more you're going to need to wash your hair. If you shampoo less often, your body will recognize that it doesn't need to produce as much sebum as often, and will dial it back.
This isn't always the case, and science hasn't yet studied the phenomenon enough to determine whether or not it's actually true or just anecdotal evidence. Still, it might be worth trying; dial back on shampoo for a month or two and see how your body adjusts.
Do You Have Scalp Issues?
There are a lot of different issues that can involve the scalp. 
Since the scalp is basically a specialized version of skin, many skin issues are exacerbated in the scalp areas.
- Dandruff. Dandruff, or flakes, is simply a buildup of dead skin on the scalp that doesn't slough off as well naturally and builds up more. It's not dangerous, but it's itchy and can be bothersome. It's also a complete mystery why it happens. It's been studied, but no one really knows for sure. That said, if you have dandruff, you typically want to shampoo more often, usually with a medicated anti-dandruff shampoo.
- Seborrheic Dermatitis. This is a much more severe version of dandruff, which causes reddened scalp and a lot of excess oil. It can even lead to crusty and greasy scales, similar to psoriasis. People with this condition probably want to shampoo every day with medicated shampoos.
- Ringworm. While not actually a worm – it's a fungal infection – ringworm can infect the scalp and will cause a variety of issues. It's also contagious, but temporary, so treating it while you have it is important.
- Folliculitis. This is an inflammation of the hair follicle, and can lead to excess oil, irritation, acne-like symptoms, and other side effects. Treatment usually involves keeping it clean, but sometimes the cause is something like shaving or plucking, which needs to be avoided until the area heals.
- Psoriasis. Care for psoriasis is very specific. The condition is basically an extreme version of seborrheic dermatitis, an extreme build-up of dead skin that causes plaques, scales, and other issues.
So, generally, if you have scalp issues you're more likely to want to shampoo on a daily basis. The longer you let your hair go without shampoo, the more likely those issues are to get worse.
Is Your Hair Long or Short?
People with longer hair tend to not want to shampoo as often, because it's a pain to dry afterwards. There's also the consideration that it takes more shampoo to clean longer hair, and as such can be a more expensive habit, albeit barely. Longer hair also tends to get clumpy, stringy, and more gross-looking if it's left unwashed, whereas shorter hair doesn't have the space to show such issues.
Is Your Hair Thick or Thin?
People with thicker hair tend to have more resilient hair. It can withstand more frequent shampooing, which usually tends to dry out hair and make it more brittle. Conversely, thick hair is even harder to thoroughly clean and takes much longer to dry, so it might be less viable to frequently shampoo.
Those with thinner hair might consider shampooing less often, to see if it helps restore the natural sebum that protects the hair. It's possible to restore hair to a thicker baseline, though it won't be a hugely significant change. On the other hand, thinner hair gets stringier and more obviously greasy when unwashed, so the more appearance matters, the more often you should shampoo.
It's also worth mentioning here that those with curly hair may find it beneficial to hold off on shampooing every day. Curly hair tends to be more resilient on its own.
Is Your Climate Humid or Dry?
External factors can affect a few aspects of hair.
Dry climates with low humidity tend to dry out hair. Dust in the air can settle and make the hair dirtier, the scalp itchier, and increase the risk of scalp issues like dandruff. Dry hair tends to be more brittle. You can help protect your hair with shampoo, but it's more likely that you'll want to use a conditioner or a natural shampoo alternative instead.
Those in a moist climate, on the other hand, are more likely to experience a greater build-up of oils and grease. Damp atmosphere leads to reduced evaporation, which means sweat – and the grime carried off by sweat – lingers on the scalp more readily. This makes it a lot harder to keep clean without regular shampooing.
Do You Exercise Frequently?
Exercise is another reason why you may want to wash your hair more often. The more physically active you are, the more you sweat. While we're primarily used to thinking of sweat as a pits kind of thing, you have sweat glands all over your body, including the scalp. 
Sweat builds up in the hair and adds grime and dirt to the oils in your hair. Since you're already cleaning off after you exercise, a shampoo might be ideal. On the other hand, it might not be necessary even then, especially if you're not naturally a very sweaty person.
What Kind of Shampoo Do You Use?
There are hundreds of different shampoo products, shampoo and conditioner combinations, and everything else. Some are medicated. Some are all organic. Some combine ingredients from multiple different kinds of hair treatment, to help retain or replace oils, or to protect hair dye and styling.
If you're using a medicated shampoo, chances are there's a reason why. That reason usually means you're treating a scalp issue, which in turn usually means you want to keep that treatment going. In these cases, you want to shampoo more frequently.
If you're using an organic shampoo, you're probably more concerned about the natural oils and resilience of your hair than you are about the deep clean. That's fine, and it means you're more likely perfectly able to go several days without shampooing on a regular basis. There's nothing wrong with avoiding harsh chemicals and oil-stripping treatments.
What Styling Do You Do?
Various kinds of hair styling, such as heat treatments and hair dyes, can have varying impact on your hair. Heat treatment tends to strip any oils already present, and to maintain the treatment, you probably want to avoid washing it right away. Dyes can fade when washed, though they're more likely to simply fade with age and UV exposure. You can shampoo as desired using a dye-protective shampoo.
So, Really: How Often Should You Shampoo?
The lower end of shampoo is once a day at most. This is true 99% of the time, and the exceptions are generally doctor-recommended. For example, if you have a hair infection like ringworm, or you have something like lice caught from your child's summer camp, then you're going to want to shampoo as often as your doctor tells you is necessary to treat the problem.
Once a day is appropriate for the people with oily hair, the people very physically active and sweaty, and the people who have scalp conditions like dandruff they want to keep under control.
Once every other day is the sweet spot for many people, particularly those with middling-oil hair and those with shorter hair. When you don't have a specific problem you're treating, and you just want to keep things under control, shampooing every other day is fine.
Once every three days is also a sweet spot, for people with longer hair and people whose hair is less oily. These people can generally go fine for a while without needing to shampoo. When there's no pressing issue and no external forces getting your hair dirty, it's fine to go three days between shampoos.
Going four days between shampoos is usually only reserved for the people with the least oily hair, and people who are trying to protect their hair treatments. We usually find that this is pushing it, but your experiences may vary.
Going five or more days without shampoo is pushing it for most people. Very rarely there may be a reason to do this, but it, like more-than-once-daily shampooing, is also often a medical issue. For example, if you had surgery and have an open wound on your scalp, you probably don't want to shampoo it every day. Obviously, follow your doctor's orders here.
There's no hard and fast rule for how often you should shampoo, so change it up and see how it feels. If you think you're shampooing too often, feel free to dial back for a few weeks or a couple months and see how your body adjusts. You might find you can get away with less than you previously thought, once you're used to it.
