McJim - my 2c:
Tyres deteriorate in quality at different rates, depending on their composition (cost?). When I've bought cheap Chinese tyres in the past I found they tended to harden up after the second year (if I hadn't already binned them because of shit handling/grip).
On the other hand I had a ZZR1100 with a Bridgestone (BT010?) I rarely used some years ago, and the tyre on that bike lasted at least 5 years until I sold the bike. It didn't appear to loose any of it's original handling qualities during that time.
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