useful life of street/R compound tire

[tnord]

from another thread:

the fact of the matter is, a tire's useful life is about a year. the sun, temperature changes (heat cycles), and just pure age deteriorate a tires ability to grip. even though you may still have enough tread depth, the tire just isn't what it used to be. i change my tires pretty much ever summer (of course i trade cars about every summer as well).

the "life" of a tire may be even shorter than that.........i dunno. all i know is my tires sure as **** don't grip like they used to.

which gives me an idea.......

that idea was to ask you smart guys how long tires last. i'm not so much concerned with mileage, but how many heat cycles, or how long (as in months), a sticky street tire will last.

what about R compounds? do heat cycles effect them as much? what about the sun etc.......

the book i have didn't give anything close to an answer, or rule of thumb.

*cue mike gerst