this is something I've often been unclear on. My Axo is about a foot long at one year old, and his appetite does not seem to have diminished.
The staples I feed him are beefheart, bloodworms (not tubifex), and brineshrimp, with the occasional feeder guppy every week or so. If I feed him anything less that a large mouthful a day, by evening, he will be hungry again. I feed him daily, with his water temp at about 63 F / 17 C.
I don't know if this is considered good practice for his age/size, but he doesn't seem to be complaining...