agree with the ones that said for a 6 pound flat that was in there way too long.
check the brisket howto in the roadmap a weath of information. internal temp if you are paranoid like me, or sliding a probe in and it should be like going through warm butter, are the ways to tell if it's done. Though I've known people that can touch the outside and tell.
the very loose time guesstimate on low and slow for a brisket is like 1-1.5 hours per pound. But it is always trumped by your doneness checks.