With proper ground prep and basic maintenance, you shouldn't be looking at anything major for 15 to 20 years or more. The pressure-treated structural components carry a lifetime warranty against decay and termites. The metal roof will outlast the shingles on most houses. The framing is solid and built to last.
The things that shorten a building's life are all preventable: sitting on bare dirt, letting moisture collect underneath, ignoring maintenance on wood siding, or letting debris pile against the base. Take care of the simple stuff and your building will be standing long after you've forgotten what you paid for it.
