Longevity of flash memory

I know that CD-R and DVD-R media degrades over time, even without use (though I don't understand why). Does anyone know how long I can expect data to remain uncorrupted on a USB flash drive? I've found lots of references to the number of times it can be written to (10,000 to 100,000), but nothing on longevity of the data when the drive is not being used. Can I put data on a thumb drive, put in a safe and come back in 20 years and expect to be able to access the data?
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Chrysler Slant-6 Engine History

Technically, no.

I read somewhere that the life of data on those types of devices is somewhere around 2-5 years, but I could be wrong. I'll see if I can dig up that article again.

Thanks very much, appreciate the help. I can't seem to find the answer anywhere.
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Csi dicussion

Flash memory is only about 20 years old, so no one has really had a chance to test this. I guess they were more concerned with read write capabilities than data longevity. My first flash drive has maintained the same data for at least five years.

Xpirate: I dont doubt that at all. Since flash memory is "non-volatile", it's supposed to remain indefinitely.. whether or not thats true, I'm not sure.