If you can keep your temps in that range I would say 4 1/2 to 5 hours, however, my experience trying to BBQ on the kettle is that it is difficult to keep temps in that range for extended periods of time.
Not saying it can't be done, just my experience. So to be on the safe side I would say 4-5 hours, less time if your temps run higher.
Treasurer, Great Lakes BBQAssociation
Rubbed, Smoked, and Sauced Competition BBQ Team-
22 1/2" WSM, 2 18 1/2" WSM's, 22 1/2" Weber Kettle