It really depends a lot on the fish that you are keeping, and their age. Juvenile fish will suffer if they don't get enough to eat, slowing growth and weakening them. Predators need less food--a diet high in protien provides more energy than one with more vegetable matter. And of course, temperature plays a key role in determing how many calories a fish burns in a day. Coldwater fish of many species can literally go for months without food--some tropical species can go for weeks quite comfortably if they are in good shape.
So--for an aquarium--most tropicals will be fine with every other day feedings. The best guide is the fish themselves--not behavior, because most fish will always eat, but rather condition and health. I feed most of my tanks every other day. Those with juvenile fish get a bit more, those with predators get a bit less.