On my car trailer (225/75 R15) I have been averaging 18 months on a set of tires for the last 8 years. Out of four one would separate or start cracking so I replaced them all. I keep the tires are covered when not in use. In 2012 I got a set of the new Carlisle tires. I have had bad luck with the old Carlisle tires so I was reluctant to get them but I wasn't going with Tow Masters again. It's been 24 months and about 4000 miles and the tires look new. I now remove them and store in the garage when not in use.
So far no issues but I still carry two spares.