It's all up to you. You can feed BP's mice for their entire lives ( might take a few per week) or you can try switching to rats. If you really want to switch to rats, I suggest to only offer appropriate sized rats, every 5-7 days for juvies, 7-10 days for adults. Do not switch back to mice even if he refuses to eat ~3 weeks (pending on the health and fat reserves the BP has to begin with). If your BP gets hungry enough, he will eat it. A little trick that you can try if he doesn't eat the first rat offered, put mice bedding in with the rat for 30 min-1 hr to 'scent' the rat. This usually temps the snake enough into eating said rat.
Some BP's completely refuse to eat rats, so it's really a judgement call as to when to give up trying to feed rats and just going back to mice.