sounds like you need to do some interval training, haha.
my favourite way to hit BP is to do it in pairs. you hit 10, then your partner hits 10, then switch back. if i just sit there and hit 30 straight, that's enough to start messing with my swing if i'm working on power. doing 20 hits in a round, and then waiting 20mins until it's your turn again sucks just as much though. the first 10 end up being a warmup, and i only get like 10 real, game type swings. i'd hit 1000 hits a day if i could. i just love to hit. but if i was trying to actually improve instead of just have fun, i'd probably do the 10 on 10 off thing for maybe 40-50 hits a turn, and 2 or 3 rounds. i don't think you need to work on your stamina for swinging a bat, so it'd actually be detrimental to develop that kind of muscle. same reason sprinters don't usually run 10miles for a workout