Believe it or not I just did 8 butts for a graduation and my uds ran for more than 24 hours straight on 1 full load of charcoal and about 6 or 8 fist sized or smaller chunks of wood mixed in. I'm guessing between 20 to 25 lbs of coal, there was some left from last time and I just tossed in a partial bag and a whole bag of kingsford.
I did the 2 larger ones then 3 and 3. I had the 3 waiting on a grill rack so all I had to do was open the lid, take out the rack with the cooked ones, set in the rack with the uncooked butts and put the lid back on.
I could not believe ther was even some left at the end although the temp was starting to drop a little with all the vents wide open.
very pleased.... so were my guests.