You need an internal thermometer because you can't guess at this. The meat will cook and then there is a period when the connective tissue/fat is being converted to gelatin/juices. The temperature will not rise for a very long time. This is called the plateau period. You have to watch carefully because once everything is converted the temperature will start to rise quickly and you can easily overcook it (holds more true for pork butt), so wait for your desired temp, pull it, and let it rest. Yes, 2 hours is the minimum for brisket. Some wrap in foil, others say it ruins the bark and it is still cooking. I find this true, the best method is in an Igloo type cooler. It is a large cut and the resting allows the juices to redistribute, just like in a steak, but think of the size difference here and you'll understand the time difference.
Keep in mind it can plateau anywhere from 180 to 205. There is no real target temperature for doneness, pitmasters have the fork twist method, the shake method, etc., but you'll only know how to judge by those methods after many tries. For newbies, poke the brisket with a skewer or the probe of the thermometer, and when the meat is tender it will slide in and out with no resistance. This will usually happen at 180-90 but some pitmasters cook longer for tenderness. Believe it or not you can cook it up to 205 and it will be more tender, but it will start to fall apart, which is okay for sandwiches, but really you should shoot for the 180-190 window, especially for nice slices, assuming it has plateaued.