Many people, especially those with oily hair, actually wash their hair too much.  The right frequency of washing depends on several things.  Oily hair does need to be washed more often than dry hair.  But washing it too much can strip away too much oil and dry the scalp and hair out.  It can also cause hair breakage. 

Most people, even those with oily or greasy hair, can get away with washing every other day.    The only time it might be necessary to wash every day is for a very active person.  In this case be sure to use a gentle cleansing shampoo. 

Those with dry hair can go even longer.    You shouldn’t go too many days between washing however, because moisture does need to be added back to dry hair regularly.  Using a good conditioner every few days can make a big difference in how your hair looks and feels.  How often you wash your hair is really not as important as using the right ingredient.  Using proven products that suit your hair type will make your hair look and feel better.