I go by color. It usually takes 4 to 5 hours @ 225 - 250 to get it looking like this. I spray it once an hour with 1 part vinegar to 4 parts apple juice. The acid in the vinegar seems to keep the outside from getting tough.
Then I put it in a pan with about an inch of apple juice (and whatever spritz you have left) and cover with foil. Crank the heat to 350 or so. The probe test will tell you when it's done. Usually another 3 to 4 hours or so. It's so tender by then I have a hard time cutting it because it just falls apart. Then I pour the juice from the pan in to a big glass beer mug and let it sit for a few minutes. The fat will rise to the top making it easy to scoop out (DO NOT put it down the drain). Then take the rest of the juice and pour it over the sliced brisket. My family can't get enough if it.