Matt,
I had abandoned the Minion method for most of this year's cookoffs, but decided to return to it when I found that temp control was too challenging.
Earlier this week, I had the urge to cook some ribs, so, while I did not completely fill the ring with charcoal, it was about 2/3 full. I lit exactly 15 coals and made sure they were fully engaged before spreading them over the unlit coals. I added 3 chunks of hickory, right on top of lit coals. That was at 7:15pm. I ran the temp at 220-240, with just one vent about halfway open. I took off the ribs at 12:30, and the next morning, at 8:30, the smoker was still going strong at 219 degrees.
Without trying, that was 13 hours. I used Royal Oak charcoal (not lump), weather was calm and mid 60's.
I asked my wife to check the temp at noon, it was still reading 202 degrees, and if I had opened other vents, I probably could have been in operating range for 16 hours.
It does work, probably longer with lower temps, but it does work.
Dale