I do not trust a flash drive with any important data. Some flash drives claim they can survive up to 10,000 accesses; others - usually cheaper - can only handle 1,000 accesses. As far as a lifespan in years, I've seen 10 mentioned quite often. But keep in mind that is (1) pretty much a guess and (2) an average. So some lasted longer and others died sooner. My advise for storing critical data is to create several back-up modes, including online storage. If you do use a flash drive, store it in a cool place with low humidity.