I just cooked an 11 lb. whole packer on Monday. This took me 19 hours to make. If you are going to prepare your packer by marinading, rubbing and letting it sit, or injecting, you should factor in those hours as well including resting time. Mine was cooked on pellet smoker at 225 degrees until the internal temperature hit 200 degrees. I recently posted a thread on this which included a video on Tuesday, 9/23/13.
There will be others who will tell you about their methodologies that work for them, and this may be better for you. There is a lot of discussion and approaches for a successful brisket including the selection of the brisket itself.