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At 250 you should figure on about 2 hours per pound just to be on the safe side! (that's 28 hours) Or you can crank up the temp and get it done a little quicker, just be sure to carefully monitor pit temp and internal temperature of the meat.
The key; however, is to cook to an internal temperature of 200 degrees! That is the point where the brisket should be most tender. Test it with a toothpick to be sure though...if it slides in and out like hot butter then you are ready!!!