I would figure at least 8 or 9 hours to be on the safe side. Last one I cooked on my UDS at 300 took about 10 hours total to probe tender. Foiled at 185 reading in the thickest part of the flat. Total weight of brisket was 12 1/2 pounds pre cook. Good luck!
UDS, cheap o grill, rusted chimney, yeller wannabe thermo pen, Maverick ET 732, 1950 model wife.