I go off of the type of fish that I have. For example, I feed my africans twice a day, my planted platy tank twice a day, my oscars once a day (skipping a day at once a week), my planted community tank with gouramis, angel-tetras-cherry barb-etc. once every other day. I feed the cichilds in this manner because it helps keep aggression down, the platies this way because there are fry in the tank, and the community in this manner because they are all adult fish.
Fish are oppurtunistic eaters. So long as your fish are active with filled out bodies your feeding schedule can vary to suit your fishes needs. (All depending on the species) Fry will need to be fed at a minimum of twice a day, adults once every other day, and juvie feeding determined by there species. If you are having food left over, cut back on the amount. If all the fish are healthy and active and eat the food in a few minutes then I would say you've fed enough.
