How long should tires last before they dry rot?

I replaced six Michelin tires a year ago after a number of years of non-use. Some were 4.5 years old some were 8 years old, all had enough sidewall cracks to be suspect. The Socal sun, summer temps, sitting mostly undriven and not using tire covers seems to shorten tire life considerably. Wish we could get some cold hard facts on RV tire life not confused by hearsay or brand loyalty.