revisions...how many is to much?

i realize that revisions are part of designing, but how many is to much and how do you charge when it becomes excessive? Every once in a while i get a client with the..."let's try this, or "what about this"...till it drives me crazy. revisons are normal, and i usually always oblige, even after it gotten to the point of excessive, but how do you tell them that enough is enough, or how do you stop and tell them that if you want to keep trying different "looks" that it will cost more? what is normal to charge for revisions. thanks for any input