I've held pork butts for as long as 12 hours in a 5-day cooler with additional blankets/towels to fill up the space. They were always above 140 when I got ready to pull them. Butts need a few hours holding to get real happy!
At comps we regularly hold the butts and brisket 4-6 hours. We pull or otherwise prep the turnins at turnin time...they are always too hot to handle without gloves.