Do not cook to time, you have to cook to temperature/tenderness with Brisket.
I've had 11-15lb briskets take 7-8 hours and I've had some take 12+ hours.
So many variables w/ heat etc that its always best to stick to your temperature and use your probes accordingly.
I usually go 225/250 for a slower brisket or 300 for a "hot and fast"
Wrap @ 170 and then let it go the rest of the way, it will stall/hold during this time but leave it be until somewhere from 205-210 depending on your liking.
Let it rest for 2 hours + (I never let it dip below 150 after resting)
Took me a few times to get this figured out on my smoker, but since then I've nailed just about every one.
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