3 times a day and as much as they will eat in 3 minutes. also get them some krill and brine shrimp. they should eat beefheat from your LFS becasue its already processed and it is easy to break apart in the water.
make sure you vary their diet as hastatus will tell you, try brine shrimp and krill and beefheart as well. Don't give them the same thing everyday. Brine is better for them than the bloodworms, and some say krill helps with the coloration of your piranhas.