I usually smoke mine for eight hours and then double wrap it in foil, and put it in the oven (set at 225) for another 8-10 hours (usually overnight).
Most people will say to cook by temperature, not time, but I've had very good results with the above method. (Not crispy bark, though, which is a problem for some people.)
You can smoke it at 225 for a long time, or 275 - 325 for a shorter time. Butts are very forgiving!
Just be sure to understand that you may encounter a stall when the juices start steaming, especially at lower cooking temps. Don't freak out and pull it off the smoker - it isn't done and it's not getting dried out or tough!
Many ways to successfully smoke a butt!