I am usually smoking butts in the 6-8 lb range @265-275 and I plan on a 10 hours on the smoker. So I would think 8hrs +\- would be a good guess. As far as internal temp, I always pull mine off when it hits an internal temp (taken at the center of the largest section of the roast) of 190*, double wrap in foil! double wrap with hand towels and let it rest in a small cooler for at least 45 minutes. Another way to tell if the butt is ready to come off the smoker, is when you insert your temp probe, it should slide in and out with little resistance and come out clean like a toothpick when a cake is done.
Good luck, and I hope I've helped a little bit...
Tim