I wash my hair once a week...I've never needed to do it more often. But if you just get your hair wet without shampoo I don't consider that washing. More of just getting it wet like when you swim.

Some people have to wash their hair less often than others, it depends on how oily it is. When you do need to wash it, shampoo is necessary to get rid of the bacteria and dirt off your scalp. It also will make your hair look nicer in general.