If you cooking several butts, and need to estimate your time to finish, take the largest one, multiply its weight by two (2hrs/lb), and add a "little bit". For 2 butts, that might be 1/2 hour, for 4, maybe 2 hours or more. Conditions will make a difference. The beauty, however is the hold time you have with the butts when finished cooking - at least 5 hours if wrapped in HD foil and placed in an empty cooler with towels.
Paul