i would guess about 6 hours if its reheated to 140. i would be afraid they would start to dry out.
but if you mean a just cooked one, pulled at 190+, wrapped, coolered and then maintained once it reached 140. I would say 12+ hours.
i have had 2 briskets just pulled from the pit and sit in a cooler overnight and 8 hours later, they were at 143.