I used a charcoal grate for a Weber 22.5 kettle with sides that are 12" tall. I can easily go 24+ hours at 275* using lump. I've never actually let it go all the way out, but I'd be confident that it would go at least 30 hours.
I just built another UDS and used a grate from a Weber 18" kettle. I haven't used it yet, but like it better already because it will be easier to pull out and put into the drum. I'm guessing it'll go at least 20 hours once I start using it.
Homemade trailer Roaster a.k.a "Hog-A-Nator"; UDS; Dual Chamber Charcoal; ECO Gasser