It depends on what temp you decide to cook at. If you go with 225, I'd go closer to 16 hours if possible just to give yourself some wiggle room so to speak. If they finish early, don't sweat it, just double wrap in foil and let them rest in a towel lined cooler. That or heat your oven to about 200 or so, turn it off and let the butts rest in there. After the butts have rested a spell, then start pulling them.
It's better to have your butts finish early than to have a bunch of people waiting for a stubborn butt to decide that it's finally ready.