Most guys I know who use the trailer-mounted Jambos at comps go about 45 minutes before adding another piece of wood. The Jambo pits are things of beauty and I would love to have one some day.
Not to add confusion to the discussion, but if you are set on an offset take a look at the Peoria cookers. they have a 24 x 48 backyard model that has an insulated firebox option.
Also, R&O makes some nice looking pits. I don't think they have a backyard offset but can probably make one.