I clean every time I shoot, whether it's 10 rounds or 100. I'm a little compulsive that way, but I can't stand the thought of a dirty gun. Besides, the gunk can build up pretty fast and I can't imagine that it's doing the gun any good.
I find that cleaning is actually relaxing. Also, it gives me a chance to thoroughly check out the gun for loose screws or any other problems that might be apparent to the naked eye.