It is damaging to wash your hair every day if you are using sulphates/shampoo. If you're going the co-wash route it shouldn't be so bad ... still though, you are stripping your hair of its natural oils.

I wash my hair twice a week and that works fine for me. But, then again, everyone's hair is different. You just have to experiment and see what works for you. But like I said, if you're not using shampoo and are simply cleansing with conditioner, then I think you should be okay.