A general rule of thumb for cook time for most any size of brisket is 1.5 - 2 hours per raw lb. of meat. This is always only a guess, because every piece of meat will be different. Sometimes they'll get done in 1 hour per lb., sometimes it might take 3 or more hours per lb. I like to smoke briskets @ around 250*, plan on 2 hours per lb., and then add a couple of extra hours as a margin for error. The larger the cut of meat, the closer I would plan on the higher cook time. If it finishes early, it will rest nicely wrapped in foil and a towel in an ice cooler for several hours.
One tip that I like is to smoke the meat the day before, then on the day of your gathering you can relax and take the guesswork out. Briskets reheat very nicely the following day, and that would allow you to enjoy your company instead of worrying about the food being done in time.
Good luck, and please share how it turns out!