I use a large BGE. I usually smoke a butt straight through, put it on let it smoke for 1.5 hours or so a pound. I want to wrap my butts in foil also. I put goodies in the foil to enhance my meat: grape juice, cherry juice, etc. How long do I wrap in the foil? I planned to cook for about four hours smokin, then wrap in foil, then back on the smoker for about an hour to tighten the meat back up.
My general rule of thumb is to cook the butts (or anything else) until it looks real good and browned before wrapping. Then wrap and cook to desired doneness which isn’t done very well by timing. I like to cook my pork butts to an internal temp of 195-200. I’d go all the way to that wrapped in the foil and then serve them. If you want to return them to the cooker unwrapped you’ll probably want to do that when they reach an internal temp of 190.