Yeah agreed 20 hrs is a long time for a brisket cook- man why you wanna punish yourself like that?! Lol
Personally, I usually end up cooking about an hour a lb give or take, at 275ish. I mean, its done when its done, but that's typically how long it takes me.
Agree with others that finishing in oven will be fine and allow you to save some coal. Shut it right down after you pull meat and it'll snuff out. Coal can be reused-hell I never fully dump my basket, just shake out ash and add more coal to the half used coal already in there.
Okie Joe offset, UDS, Akorn, Weber Kettles, I'm JD.