"Total time 6 hours"
I recently made this mistake with just a flat. Thinking that because the piece of meat was smaller (than the packers I usually cook), I didn't think it would take as long to cook.....I WAS WRONG! (My wife loves to see those three words).
What I learned was that regardless of the size of the brisket, because it is one of the toughest cuts on the beef with the most connective tissue, only time at temp will break those connective tissue down into the tender delicacy we all look for.
In my case I put mine back on the smoker for and additional 3 hrs +/- until it was done. I spritz my briskets with apple juice about every 1-1 1/2 hrs and keep a pan of water in the smoker for humidity.
My guests had all gone home by the time it was done thinking "that was like shoe leather". However, I had a great late night snack and learned a valuable lesson.
Lesson: A BRISKET IS ONLY DONE WHEN IT'S DONE.