how often

I have read many articles by googleing......but couldn't find the fact....how often we should shampoo our hair, some group says we can do it daily,while some says washing your hair daily with shampoo damage the hair......i shampoo my hair every 3 days(or 2-4 days ) as its long till my butt plz dont laugh but its true that i dont know how to use conditioner , after wash i just apply it on my hair then count 1-60(1 min) and just rinse off......is it the way to use conditioner ........dose any of you using Kerastase , i want to give it a try but its so expensive so befor i get i want opinion is it worth to spend that big bugs....dose it really make you feel different then other inexpensive shampoos