While it does not qualify as a "single" cook, this past summer, I kept two WSM going non-stop for 9 days at a state fair in Alaska for a BBQ booth we were running.
I was cooking 6 butts at a time on each and half way through the cook, I could consolidate 4 on each rack, freeing one up for another 3, so at times, I had up to 8 butts in the WSM.
As the meats ended up being on different schedules, I just kept refreshing the coals about every 15 hours-or whenever I could not maintain 250 degrees. I only had to use my chimney for starting charcoal twice--the very first time and about the 4th day or so.
What I did was set up a Weber Kettle to hold the ashes. I would remove the top part of the WSM and dump the coals/ash right on top of the Kettle cooking grate. I would refill the WSM with unlit charcoal and using tongs, place some of the still hot coals from the kettle right on top. Worked great for us.
I guess it was a virtual Minion method.
In terms of longest cooks, I did 22 hours once. 18 for a brisket and 4 more hours for some chicken before I had to add a few hot coals to the pan.
Dale