I don’t use a UDS, but your cook temp will drive your time. So it depends on what you temp you can run stable for a long cook.
On my Bubba’s at that size I’d say..
200-215* = 12 hours +/- 1 hour
225 = 10 hours +/- 1 hour
250 = 7 hours +/- 1 hour
Double foiled, wrapped in towels and stuffed in a cooler you can hold it above 140* for several hours easy. I’d shoot for getting it done a bit early to be safe and use this to transport it. Slice at her house.
That said listen to the UDS pros, not me. They know your cooker better than I. Oh, BTW brisket is tricky. Taking your first brisket to your grandma’s 89th Bday shows real guts!!! I’ll be rooting for ya..
Go fast ~ Cook slow...
2 Bubba Keg Convection Grills
4 Big Steel Kegs