There is no fixed amount. The common procedure is to place a small amount of goldfish food on the surface of their water. The fish will usually swim to and eat it. If the food is all eaten in less...
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...
A goldfish will only eat as much as it needs. It will stop eating when it has had enough. It is impossible to overfeed goldfish. You should give a goldfish small amounts of food for up to 15 minutes....