Once a day as much as they can eat in 3 minutes, or twice a day as much they can eat in 2 minutes.
Their stomach is about the size of their eye, so that's a guide as to how much will fill them. Feed a little, see how long it takes them to eat it, if they polish it off in 1 minute then add more. If there is still food after 3 minutes then you have fed too much.
If you have bottom feeders like Cory's, then you need to make sure something gets to the bottom too. They will scavenge throughout the day and night for bits of leftover food too.