** I misread the question when I posted this. As others who answered the real question said, remove when meat temps reach 195-205 degrees **
Remove the butts from the WSM, wrap in foil, and let it rest. If you place them in an empty ice chest in towels, it will stay hot for as long as 4-4.5 hrs. Pull just before serving.
If the time between the end of the smoke and dinner time is long, consider inserting a remote probe thermometer in the meat so you will know if it drops below 140 degrees and the risk of bacteria increases.