Having grown up in Texas, I learned to BBQ brisket from my grand dad. You have to have an untrimmed brisket. The market trimmed briskets do not have enough fat to carry through the meat and keep it juicy. The second factor is making sure your water pan does not go dry. If you have a good smoker with a deep water pan the meat is basically steamed. The water pan resides directly between the meat and the fire. A 15 pound brisket should take 16-18 hours. I use small wood chips soaked in water with a propane smoker at 225* . I typically marinade it for 48 hours in claudes brisket marinade. It always turns out fantastic . Always cook it with the fat side up!!