I've found that the small briskets I have done took almost as long as a full one would. I've done small 3-4lb flats and points, cooked to around 200* IT, and they all took 9-10 hours. Someone suggested that is more about the thickness than the overall weight. I think that makes some sense.
I would hedge my bets on it taking longer than expected. It will hold wrapped in foil in an ice chest for many hours, or you can always use the smoker as a warming oven.