It depends on how well they are done but I wash mine 2-3 times a week. I use MTG so I HAVE to wash often.
But, I go to a good braider so I can wash often without a problem. I also get touch ups every 2-3 weeks around the edges and all redone after 6-8 weeks. If I did not do that I am sure that washing so much would not be so good.
Also, I do most of my washes with a stocking cap on and I dilute the shampoo with water. Once a week I do a wash w/o the stocking cap.