If you have really oily hair then you should wash your hair daily or every 2 or 3 days per week.
However if you have dry hair and non oily hair then you can and should only wash your hair around 2 times per week.
Washing your hair too much especially when you have dry hair can dry out your hair more and damage the hair follicles.
You can usually get by washing your hair once per week if you have dry hair otherwise washing your hair too often can remove the natural oils in your hair which can dry out your hair and dry out your scalp.
I wash my hair twice per week unless I happen to get dirt in my hair somehow.