1. Skin type. If your skin and hair are anywhere from normal (not super oily and not super dry) to dry, you probably only need to wash it once or twice a week, according to a Columbia University health column.If you have a greasy scalp, you probably need to wash your hair more often. 2.Hair texture. Texture matters because it affects how quickly sebum works its way from your roots through the length of your hair.. Skin type. If your skin and hair are anywhere from normal (not super oily and not super dry) to dry, you probably only need to wash it once or twice a week, according to a Columbia University health column.