Hope you're planning on doing the overnight approach. I've done the same cook and it took me just over 15 hours. Having said that, I just finished cooking one butt and two picnics and the butt took 21 hours. Perfect weather, great temps, just the butt didn't want to get tender. I was planning on 14-15h for the butt. Don't forget that both butts and briskets hold well in HD foil wrapped in towels in a cooler (I've held butts for 4 hours and it was still hot enough to be very uncomfortable while pulling).