Spare tire lifespan Continental CST 17

[FireFox31]

What's the lifespan of the doughnut spare tire which came with my 2013 XC70? It's a Continental CST 17, manufactured in 6/2012, and had no apparent cracking or dry rot. I don't see lifespan listed in the owner's manual and can't find anything from Continental.

My local tire shop said "the rule for spare tires is '50 50'; drive under 50 mph and replace after 50 miles." A Tire Rack article reads slightly differently, suggesting that it shouldn't be driven more than 50 miles in one trip (perhaps due to heat build up). I can't believe a doughnut is only good for 50 miles ever. I'll appreciate any information to clarify this. Thanks.

[abscate]

My local tire shop said "the rule for spare tires is '50 50'; drive under 50 mph and replace after 50 miles.

I think these guys are confused. The user manual usually says something like 50 mph and under 50 miles per trip, this may also be related to the offset tire size on an AWD veHicle

Temperature wise, a tire will reach equilibrium pretty quickly in summer, like 10 minutes, based on my limited measurements.

[BlackBart]

Oh, this all reminds me that the skinny spare in my 850 is dry and cracked. I used it only once, to get two miles to the tire shop. I really doubt the first owner ever touched it, so that’s almost a 30 yr old tire, yike.