Stug: Military, supply ware houses and parts distributors: shelf life is put in place for rubber parts to allow parts in stock to be rotated in a timely method to be issued for use. Oldest parts issued 1st and new parts are stocked at the back of the bins eventually being issued before the shelf life has expired. Just like food in your grocery store. This applies to most rubber parts, or rubber impregnated parts. Specifically aircraft tires, rubber fuel tanks, rubber pressure lines, o rings (Packings) engine mounts, radio isolation mounts, elastomeric bearings (pivot bearings) etc.
Shelf life varies between different parts, shelf life is generally dictated by the manufacture. So what happends when the shelf life is exceed? The part can be inspected, tested and sometimes re-lifed prior to usege. While I have re-certified parts for the RCAF, even O-Rings (Operation necessity) parts of low value (Cost) like O-rings are usually scraped, parts like Pivot Bearings for the MR Head of a Bell 412 costing several K are sent to the contractor to be re-certified prior to use.
In-service life: once a part is installed shelf life is no longer a concern. Some parts do not have a in-service life once installed on a aircraft, ridged line clamps, O-rings, airframe trim etc, These parts are replaced on condition. Some parts like tires and hoses are given a in-service life. Tires on a fighter may be replaced after X number of landings or for transport aircraft by simply tread ware. In the RCAF, aircraft flexible hoses (neoprene and Butyl) are given a 8 year in-service life (based upon manufacturing date data on the metal tag/ or aircraft log set data). This is based upon historical data, lines are exposed to elements, rub, chafe and you want to replace them prior to them bursting.
My 33 yr military carreer I rarely saw engine mounts with in-service life values rather replacement was due to poor performance. Helicopters let you know quickly that a dynafocal mount needs to be replaced, the vibration intensity increases with speed / torque applied. Fix wing aircraft the vibration is felt throught the fuselage and can be a gradual increase. Usually inspections and functional tests catch weak engine mounts prior to the vibration cenario. Heat, oil, dirt, any petroilium product sprayed on and poking with metal objects is very un-friendly to engine mounts.
To install a exceeded shelf life engine mount doesn't make much sense to me, if you go to the trouble of removing and then installing a engine it is nice to do it once with mounts that are serviceable. Due to the nature of the Thorp's engine mounts, rotating the mounts as has been stated can extend the life. Other engine mounting designs or applications rotating is not an option usually due to shape or space issues.
Rich is correct on his Inspection details, mounts need to be inspected as a min on the annual Inspection. Replacement to me is on-condition item ~ deteroration / saging / vibration.
Hope this helps.
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If I remember right I had to "shave down" the steel spacer a bit to get the right thread clearance . My engine is an 0-360 . May be different for other applications ? 
