Hi Josh.
At a cooker temp of around 250* you can roughly figure on around 1.5 hours per raw pound of butt. This is only a guess, as each piece of meat tends to have a mind of its own, and I've had them take over 2 hours per lb. You can speed your cook time up some by cooking at a little higher temps...many will smoke a butt @ 275, even 300*. You can further speed the time by foiling the butt when its internal temp reaches around 165*; this will help power the meat through the inevitable stall. Try to allow for what Chef JJ calls "CYA time", an extra hour or 2 just in case it gets stubborn in the stall. This will allow a longer rest time at the end, which never hurts.
I don't know anything about your smoker...did you say its a charcoal rig? To lower temps, try starting with less charcoal and build a smaller coal bed.
Good luck, and be sure to let us know how it turns out!
Red