Small fish such as tetras (neons ect) should be fed as much as they can eat in a few seconds 2-3 times a day and larger fish should be fed what they can eat in 2-3 minutes once a day,large predatory fish (fish eaters) should only be fed once or twice a week.
What is important is that all the food you put in is eaten and none is left to rot,fish will often ignor food if it has been in for a while so anything that hasn't been eaten after an hour should be removed.You will soon be able to gauge how much and how often to feed your own fish if you watch them at each feeding.