I smoked pork butt last Friday for 10 hours. At the 6 hour mark I added unlit coals after knocking off the ash. That simply caused to fire to reverse directions. That's one of the things I love about kettle-smoking - adding coal is a breeze.
Last weekend, I smoked some ribs with regular KBB in my 22 with SnS. The ribs were done after 4 hours, but I had only used about 1/2 the fuel in the SnS. I left them going just to see how long it would maintain temps after I took the ribs out and it was still holding 230 after 7 total hours, with no added fuel.
If I wanted to go longer, easy enough to wrangle all the lit coals into one corner, and fill up the rest of the SnS with unlit coals like Jeff J said, and it'd go another 5 or 6 easy, probably more.
I usually go with a 2x2 when I want to hit 225. You should easily be able to get 12 hours out of that if you go 2/3s or 3/4s of the way around the kettle. But all depends on ambient temperature, wind, amount of meat, if you use a water pan, and other factors like that.
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