Hair is naturally dry on its own and relies on oil from the scalp to keep it supple and enhance elasticity. Over washing your hair can actually strip your hair of its essential moisture.
How often is too often, though? It varies from person to person. Ask your stylist. Some factors include:
-Hair thickness
-Activity level and exercise
-Products used
-Air-drying vs blowouts, curly vs straight
-Chemical processes
As a general rule, hair should be shampooed only when it’s dirty. This results in less frizz and can save you from going through conditioner at an alarming rate. There are lots of “non shampoo” cleansers out there if you simply must wash every day. My favorites: Kerastase Chroma Sensitive, WEN, and Deva Curl No-Poo. Otherwise, washing with regular shampoo about twice a week is good pretty standard for most hair types while some hair thrives being washed only bi-weekly.
Christine Rinehart 
So true! Clients are always surprised to hear this!