I just did a 13.5lb packer in my upright propane smoke hollow. Except I separated it before cooking (too long to fit in the smoker). I put the flat on the top rack to allow the fat to drip down onto the point and keep it moist. I kept it about 210 until internal temp hit 158, where it seemed to find it's stall (about 8 hours in). Then I wrapped in foil, got up to 197 in 3 more hours. Took it off, wrapped them in towels, placed in a dry cooler for 1 hour, and voila. Super moist, delicious. Everyone loved it.
Point is, 8 hours would be about right with a big enough brisket, especially if you could it whole. So the only real difference is you lose the opportunity to add chips, or whatever your preferred smoke generator is while you sleep.
One last thought, make sure you have your heat dialed in where you want and sustainable before you go to bed.