Pretty much every full packer brisket I have done has been between 12-18 hrs., so at 13 hrs. you really aren't doing to bad at all. Don't rush it, don't get antsy and crank up the temp, just keep it low and slow and let it do it's thing. Every brisket is differant some longer some shorter - just relax and let it ride. Also don't rush the resting stage, 1 hr. minimum! You do those things and you will be rewarded with a tastey chunk of brisket.... you rush it and you get tastey shoe leather.