I'm surprised about the time line too. I did a butt last weekend, 8lbs, and it took 12 hours. Like I've been told and know know; it's done when it's done. Time is only a very rough estimate of where one is in the cooking process.
BTW, I put on as much rub as the meat can possibly hold. If I had a 5 gallon bucket of rub, I'd just dump the meat in there, let it sit for 5 minutes, then pull it out, being careful to not bump any off. The bark on both the brisket and the butt were excellent.