I am putting my 20 lb turkey on at 6 am for two hours at 425, then will drop temp to 275 for three hours or so. Then into the cooler to rest (while I make the 1 hour drive to the relative). But I use a meat thermometer and will go by that. I smoked a turkey last week that took about 5 hours with 1.5 hrs at 425 and 3.5 hours at 225...(skin was ok but want it crispier). It was pretty big maybe 18 lbs.

I am guessing 5 hours at 275 straight through for 12 lbs. Maybe less (but the skin will be rubbery). If you have the meat thermometer in you can set it to cook a little fast and then drop the temp the last hour or so. If it gets done early throw it in a roasting pan and wrap that with towels and put in the cooler. The outside of the meat will be hotter than the inside and internal temps can continue to rise for a bit when you do that.