You can feed them as many times a day as you want as long as you don't feed them too much food overal. Small fish like platys, guppys, mollys, swordtails etc do better on 2-3 small to medium sized meals a day, fry need 3-4 small meals or more a day. The smaller the fish, the more beneficial small but regular meals are since small fish have shorter guts which process food much faster and thus the fish gets hungrier quicker even though it can only eat a little at a time. A lot of larger fish can easily go without food for days on end though since their size and body mass allows them to process food for longer and thus they can eat more in one go and get more out of what they eat.
You should thoroughly research your individual fishes dietry needs very carefully, since they can vary massively from fish to fish, my advise is thus only "generally speaking" and may not apply to all fish.
Imagine a fish living in a pond- it finds a bloodworm and eats it and feels relatively full. But it never knows when its next meal is gonna be, and it could need another very soon, so a lot of fish have evolved to feel hungry all the time so they are encouraged to be always searching for food because their food sources in the wild tend to be small and irregular.
In an aquarium though fish have no need to worry about starving as long as their owner feeds them regular meals, but they still have evolved to look for food all the time and feel hungry a lot. Obviously you can't be there to feed them lots of small meals throughout the entire day, so most owners feed 2-3 medium sized meals a day to make up for the less constant all day supply of critters and things fish would usually find to eat in the wild.
You should feed your fish enough to be consumed completely within a couple of minutes depending on the type of fish you have and how many you have, if there's any food lying around in the tank after 10mins you are over-feeding, so you should cut back a bit. Make sure you feed fish a varied diet, a constant diet of fish flakes is not very healthy for fish (it would be like us eating hamburgers everyday of our lives), so make sure you are feeding a varied diet suited to your particular type of fishes needs. Sinking foods like pelets, wafers and tablets take longer to be eaten, so also take that into consideration too.
