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Lack of moving parts and a decent storage environment should add up to a very long functional life. I know in the small print somewhere there are warnings that the data may become unreadable over time. This would be true for any RAM but I think it would take a very long time, assuming they are in a humidity and temperature controlled environment. Or a closed office desk. I've had SD cards that were (1) a cheaper brand, and (2) heavily used, go belly up, but I've had one fail from sitting unused.
It seems you can use SD cards thousands of times...
"Flash memory indeed has limited write cycles. However, by now it is unlikely that you'll encounter this within the normal lifetime of such a card. Usually this is in the order of 100,000 write cycles today and SD cards include circuitry to manage wear-leveling, that is, spread out writes over the storage media evenly to avoid "hot spots"—pages that are written too frequently and therefore failing early."
The SD Association says the service life of an SD card...
"...depends on how your product in manufactured. SD standards-based memory cards, like most semiconductor cards, store information in flash memory. The current technology along with normal usage typically gives the card a lifespan of 10 years or more."
Not sure what constitutes "normal usage," but it sounds as though even your oldest cards should work.