That may be too long. Use 1.5 hrs of cook time per lb, dp that would be approx 10 hrs or so. I never base cooks on time. You need to go by temp. I cooked a butt today. I was about 5 lbs. I out it on at 7:30 am and took it off at 3. I didn't foil as I wanted the nice bark. I removed the butt from the smoker when the internal temp reached 202*. I put it in a ceramic bowl, covered it w/seran wrap then placed the cover and a towel on top for 1 hr. It was very good, the bark was great. Not as juicy as foiling but by no means was it dry.
If you do foil, remove the butt when the internal temp gets to 165. Double wrap the butt in foil and add a little apple juice to the foil. Place back on the smoker until the inernal temp reaches 202*. Remove and leave in foil for 1 hr. Pull the meat and enjoy.
Hope this helps.