Allyn, there's a rough estimate out there that estimates 1 to 1.5 hours per pound. But you really can't go by that; rather you need to go by tenderness. The brisket is done when it's tender. Testing for tenderness is done with a probe such a thermometer or fork. If it slides into the 'flat' with no resistance then it's done. Always test the flat as opposed to the 'point' of a brisket as the point will always be done sooner than the flat; the point has more fat in it. Factors such as temperature and weight of meat will influence the cooking time. No 2 briskets cook exactly the same.
Paul