Though shampoo companies would probably love you to wash your hair every day, the truth is, you probably don't have to. Of course, there are exceptions: people whose jobs or exercise schedules get them dirty on a daily basis, for instance. But for other people, you can probably skip the shampoo for a day... or longer. Doing so may actually be healthier, as your scalp's natural oils won't be stripped away as frequently.
There's no hard-and-fast rule to follow on this issue, though. "It totally depends on the texture of your hair, the style you are trying to achieve and your lifestyle," says Kevin Mancuso, creative director for Nexxus. "For example, fine hair needs to be washed more frequently, whereas thicker hair can benefit from the natural oil/moisture produced by the scalp."
Generally speaking, though, if your hair texture is fine, you'll need to wash more frequently than you do if you have thicker hair. In the latter case, you may want to shampoo once weekly and just rinse your hair with water otherwise; especially if you have black, natural hair, shampooing too much can make hair too dry. And though it should go without saying, if your scalp is very oily, washing once a day might be just what you need to do.