Unlike other drugs, weed really likes to stick in you system. A fat person will hold easyer then a thin person so excerise should help get you clean. Keep in mind that drinking alot of water they will pick up on and be sus.
I though hair test stayed with you as long as the hair was produced when you had the drug in your system, ie it shows an almost day by day image of whats been going on, a snap shot in time. I could be wrong though.
But from what I gather almost noone does a hair test. They are expensive and complex. Sure they can be done, but they are unlikely.