Pork butt is almost impossible to mess up.
Step one, breathe... :)
I cook butts at 250 for about 10-11 hours, until it hits 195F internal temperature. I've cooked them as low as 200F (takes about 24 hours to hit 195) and as high as 325F. The last one I did, my pit ran cool overnight, dropped from 275 to about 230, so I had to finish it in the oven at 325 for a couple hours. Still turned out just fine, even with a huge temp swing.
Don't sweat it, you'll be fine. Just cook it until it's done--either start early in the morning and expect it in the evening, or cook overnight. In either case, just let it rest wrapped in foil for at least an hour before trying to pull it so the juices will all redistribute properly. You can hold a cooked butt in a cooler with towels practically all day, so it's ok if it's done early.