Personally I have found with my son that you should be throwing as often as you feel comfortable throwing. Do it in repetitions also if possible. By this I mean go out and get warmed up and then throw 20-30 throws working up to faster velocity. Then after 20-30 throws take some batting practice for half an hour or so. Then go back to throwing only this time instead of bullpen throwing you are doing an infield defense and throwing drill, alternating between fast hard full motion throws and short quick running throws.
I have found that this routine with my son has brought amazing results in arm strength and power. The workout almost tends to be more of a cardiovascular workout more than anything where their heart is pumping its hardest in the end doing the infield grounders and pop-ups repetetively. I go until my son finally says he is tired and thirsty and not until his arm hurts or is in discomfort.
This routine is done every other day alternating with just batting practice and a 5-10 minute throwing session on the off days. When it is all told, my son and I will have thrown about 200-250 throws per week and has batted the ball about the same amount. I have found that when batting practice follows throwing practice immediately, my son is less prone to arm pain and can throw again the next day with no discomfort.
