I look at it this way, after about 2-3 hours, the brisket is cooked. That is to say, the meat has reached a temperature at which it is reliably safe to eat. The fact it is tougher than a shoe doesn't change the fact that it is cooked.
Everything after that is for tenderness, flavor, color and taste. If it takes you 2 or 20 more hours to reach this goal, the end result is the same. After lots of reading and some good instruction, I finally learned it doesn't have to take that long.
I used to spend 12-24 hours cooking brisket, but have recently changed, and the last couple have been the best I've ever done. They were both done in 6 hrs or less, and I'd put them against anyone cooking at home. I'm doing another one this weekend, I'll run a how-to thread if I remember to keep snapping pictures! LOL