At Carrier dealerships, some service departments post a sign saying the useful life of a reefer is seven years, and that repairs made to reefers older than that may not be worth the money.
While that may be a good guideline, reefers vary in application from the ones that run most of the time on 53' trailers hauling produce coast to coast, to those on local delivery trucks, to those on trucks that actually have two (one for backup) reefers for temperature-critical freight, to expedite temperature-control, and more. Some reefers run all that time at 40F. Others may be called upon to run at -10F on hot summer days.
Given the above, my answer to your question would be, it depends on how the reefer is used and how well it is cared for.