I've tried the watching thing, w/my subs and babies that are eating live, that that's just a distraction. I leave them for about 10 min, come back and they're working on eating by that time.

I wouldn't leave the live prey in longer than 15-20 min. I figure if they haven't chased it down by that time, they're not going to. I've read here some ppl do it for 30 min.