Depends how many people watch (and click) them. There's technically no limit on how much you can earn from them, but even the most popular artists don't usually get more than about $50 every few months or so (you only earn anything at all once you hit the $50 mark; only then will a cheque be sent to you).
Basically: the better the flash is, and the more popular you are, the more money you can make. Don't expect to retire early or anything, though; most people only earn a couple of bucks a month or less.