I have a friend with a joint, has 2 Old Hickory units. He cooks briskets (flats only) for around 12 hours, give or take, @185*. He puts them on at 6 p.m., then pulls them after he arrives the next morning (6-ish), or whenever they're done. He gets unbelievable smoke rings and they are super juicy.
Either these super long cook times are just for effect, or they're cooking big whole packers at super low temps.