Once a day. I prefer to feed more once a day for the reason that some of my fish are slower and more timid than the others. So can take much longer between the food arriving and them going to the top to get some. So if I feed a good amount, the other fish haven't got time to eat all of it before the slow fish get there.
However if I were to feed a small amount twice a day the fast fish would eat it all immediately and the others would go hungry!
135 litre (35 US gallons) 6 Black Widow Tetra; 3 Mollies; 3 Bronze Cory; 1 Pygmy Cory; 6 Gold Barbs
27 litre (7 US gallons) 6 Guppies.