For what I know, the main issue of flash memories is the limited number of read/write cycles, so if you don’t read/write too many times the flash card it can be used also for more than 10 years.
Keep in mind, however, that this number of read/write cycles is rather high, so this shouldn’t be an issue. Moreover, if you will use the memory only to store data with an occasional reading, I think that the chip will be able to retain data for more than 10 years (only my opinion, actually I have no proof for this :().
Bear also in mind that 10 years is a very long time, and because of 6 months are sufficient to make an hardware to become obsolete, it is possible that in a couple of year will become available something with a better reliability than current flash memories, so right now I think that to store data in time the best way (even if not the cheaper) is a flash memory, followed by a good quality optical support (CD or DVD), and finally an external HDD.
Let’s see what future will bring us