Pork butts take as long as they want. I've had one get finished in 8 hours before, and I've had them take 18 hours before with a full smoker. Last one I did was about 10.5lbs and took 14 hours @ 225. As long as you smoke it until at least 160-165 you can foil it as I'm told the meat won't absorb any more smoke. But, I've never bothered. Pork butts are one cut of meat that has so much fat that it's really tough to dry them out. In the winter I'll smoke until 165-170 then pull them and finish in the oven uncovered or tented with foil. So it's all good.
I've learned never to "plan" a BBQ with pulled pork too closely. Best advice I can give is start early, and plan to FTC the meat until you are ready.