In 2014 a buddy of mine bought a 1984 Class A motorhome with 28,000 original miles on it and original tires. He changed tires in 2015 at a recommendation of a shop because they said rubber started to rot. So, figure tires can last 30 years while outdoors.
However, I once installed a set of old tires back in the 90's on my 87 Camaro and even though thread was well over the minimum required they would skid really bad in the rain. Turned out rubber had hardened with years and didn't provide good friction with the road surface.