Each piece of meat is different. You can use loose guidelines to know when to start checking for doneness but if you are looking for a magic time or a magic IT temp to pull a brisket, you aren't going to get anybody to agree on the exact number.
@ 250-275 degrees it will take approximately 45mins-1hr per pound to cook, you can adjust that accordingly if you run lower or higher than that temperature range. Start probing for doneness around 195ish internal temperature. The last brisket I did (13lb packer) probed tender at 212 degrees before it gave up the goods.
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