If you want to take 12 -15 hours, shoot for temperatures around 250F, maybe running between 235F and 255F, you will end up in the time range. I tend to cook around 275F up, even to 300F, and my cooks typically run 8-9 hours with at least one hour at the start running at 225F. Soooo, that is what I do.
Whip It Off, Chambers!
"perhaps...but then again...maybe not..."