It kinda depends. I did a 15 lb packer on Sunday. It was around 12 hours on the smoker. I was aiming for 250°F cook temp, but was probably more like 275°F usually.
I don't foil, brine, inject or anything. The temp of the meat was 212°F in the thick of it. The flat didn't probe quite as tender as I usually like, but we had guests coming. I didn't let it rest much more than 30 minutes. Still turned out very good, though
18.5" WSM ; Weber E-330 ; Weber Q200 (tailgating) . . . Guess I'm a Weber fanboy!