Answer:
I've heard about as much as they can eat in a few minutes, one to two times a day. I just got mine recently but I always see him swimming around grazing on algae. You also don't want to feed them more then they can eat in several minutes because it apparently encourages algae growth, and overfeeding them can cause problems with infections. (I don't really have time to find references, but google it, there are a lot of blogs and articles).