I won't yell it as MD did (but he is right), but please only use IT as a guideline. After the IT reachs 190 +, start checking it every 30 to 45 minutes with a probe or a toothpick. Probe it in a number of places on the flat portion of the meat, once the probe slides in with little resistance, it's done. Pull it off the heat, if you foiled, open the foil, let the steam and some heat escape until the IT is down to 170 or so, then wrap it back up, put it in a cooler covered with towels and let rest for at least an hour, 2 if you have the time. After the rest, slice and enjoy.
The thing to remember about briskets, they each have a mind of their own. Their mind is actually the amount of connective tissue contained in the meat, more equals longer cook times, less equals less time. Thicker pieces of meat cook differently as well. Higher fat content, longer cook times, hence the reason burnt ends are made from the fatty end which requires a couple more hours of heat than the flat. So to answer your question, no it is not done too fast. It's done when it's done.
Happy Smoking!
