My brother used to keep tropical fish when we were kids, but I never took much notice of them. But I googled for you, and it said once or twice a day, as much food as they can eat in 3/4 minutes. If there's food left after that time, you're giving them too much; if they finish it after two minutes, it's not enough ...
But I expect there are some experienced fish-keepers out there who will tell you more ...