How often you feed them is no where near as important as how much. That's where it get's difficult to judge. 1 flake per fish, a "pinch", a "small amount".....it's all very vague. One thing's for sure, you should NEVER have any uneaten food after 2 minutes, if you do, remove it pgq. Some people like feeding their fish, and in certain cases it may be the only time they see a shy fish. That's fine, but over the day you shouldn't really feed any more, just spread what you do feed out. Modern flake food is highly nutritous and it doesn't take much to give a fish all it needs to be healthy. There are lots of other things to feed. A whole host of frozen food, blanched lettuce or cucumber for herbivores, finely chopped redworm goes down well, as does cockle, mussel etc. So to sum up, it's not so much how often that matters, moreover how much. Mine get flake once a day, frozen every other night and a fasting day sunday to encourage a clean-up lol.
