Kevin,
Relax.
What you want to think is that you expect it to take about an hour and a half for each pound of meat PLUS 30 to 60 minutes to rest plus time to pull and get on the plate.
If the temp runs hot (as it sometimes does with new smokers) it may be done sooner. (In this case let her go to a bit higher internal temp, let it rest a bit longer, keep warm foiled, wrapped with towels in a cooler). If it just ain't ready when you have to pull it, foil and finish in the oven. See, two ways to handle almost anything that can go wrong.
No matter what, plan for the resting time.