I have only read that once meat is introduced to heat that it needs to be to 140 in four hours. So if once its on the pit just make sure your temp stays up. If you let your fire go out and come back at hour four and your IT is 120, you may be growing bacteria. Just use cold injection, below 40 degrees and go right back in fridge with butt and you should be ok for 12 hours.
Smoke in the Mountains BBQ Team; offset reverse flow