Two weekends ago, I had the maiden voyage for my 22 1/2. We cooked two butts and about six racks of ribs trimmed St. Louis style. I fired the WSM off using the minion method about 9pm on Friday night. It was 20 degrees in Houston so I started with a little more charcoal in the chimney. I place five good chunks of pecan among the unlit charcoal (Kingsford blue bag).
I started off with the vents open 100% and closed them down to 25% when the smoker hit 200 degrees. The cooker went up to 225-230 on my temperature probe and stayed there until I went to bed at 1am. When I got up at 6a, the probe temperature was down to 210. I opened the hatch, stirred the coals a bit, added two handfuls of new coals and opened the vents. It was still cold and the wind had kicked up a bit so I was worried about making to 10a at a decent temperature.
The cooker got back up to 225-230 pretty quickly and stayed there until 8a. It started to drop off but I wasn't concerned as the meat temp was getting close to 190. At 10a, the meat was at 195 and the grill was back down to 200+. I removed the meat, wrapped it, foiled it, and put it in an empty cooler inside. When I took it out to pull at almost 5, it was still toasty warm. After I finished the butts, I refueled, did a light cleaning and put on the ribs.
So long story short, you can make it 12 hours with little or no effort even under cold conditions where you might use a bit more fuel.
Good luck.