How long should tires last before they dry rot?

Our 2007 rig has the same brand tires as the OP. They are original with 17,000 miles and still look and smell like new. But our rig stays indoors in climate controlled storage which apparently makes all the difference.

I don't worry about the age of our tires, not only because of condition, but also because our rig is 4000-4500 pounds lighter than most when loaded for a trip. The load my tires see is significantly less than they are rated for.