If you are smoking in the 225-250 range, IMO, allow 2 hours per pound. May not need that much, but if you do better to have it available than not. Just for a reference, I did one 8 lb bone-in butt Wednesday. I started at 5 AM and it was done around 7 PM. I let it rest for about 3/4 hour and my wife and I ate, fridging the left over part. So allow for some extra time and avoid problems later. If done earlier, you can always rest longer than I did.