How often hair should be washed is a matter of preference, and depends on factors such as hair type, hair style and scalp. People with thicker or longer hair require less frequent washings as it is harder for the oil from the scalp to travel down the hair.
People with shorter, thin or straight hair need to wash their hair more frequently because of this, however they still do not need to do so every day. Hair only requires washing every day if the oil production of the scalp is very high. For daily washing a lightweight shampoo is recommended.