When I used to do low temp brisket cooks, I would figure about an hour to hour and fifteen minutes per pound.
So, for me, a 13 lb packer would finish before resting at 12-14 hours. There are many variables however. I found that the bigger they are, the cooking time was lower per pound.
I do all briskets now in a high temp cook. Whole briskets are done in about 5 hours before rest.