I regularly do up to 4 ~12lb briskets in my UDS and i've yet to go beyond 10 hrs of smoke time. average smoke time is 9hrs. If i have a BBQ for that day, i put the meat in the smoker at 6am and it's typically done by 3-4pm. i let it rest till 5-6pm and slice and serve. I am at sea level and when i smoke meat the outside temps are always above 50 degrees and 60-70+ in day.
I always read that people smoke briskets for 14, 16, 17hrs or more!! I think if i did that i'd have a ball of bark with mush in the middle!!! As i've seen on here and recommended from other pitmasters it all depends. The brisket is done, when it's done!! I think the reason i get consistent times is i buy the packers from the same store and always get 11-13lb sizes. For me, thats the perfect size.
If you have limited time here is a suggestion which has worked for me in the past. Smoke the brisket for 3-4hrs, or as long as you can allow. take brisket out and wrap good in foil and finish in the oven for the last few hrs. You still have the smoky flavor and tender meat. if you want bark, this method isn't recommended. You still need to let the brisket rest for a few hrs. IMHO it's not as good as a full smoke and i wouldn't do this if i was serving to BBQ purists!! LOL
But, I have done this were my time was limited and a full smoke just wasn't possible.