| Flash works in blocks - depending on the flash device, it can be anything from 512bytes up to several Kb. When you write to a block, first you have to erase the whole block, and then re-write it. That erase/rewrite process does 'wear out' the flash, so there's a limited number of times you can erase/write to flash (admittedly it's usually in excess of 100,000 times now).
If you use flash for swap files (and log files), where heavy access will occur, it's hard to know how long it'll last. My personal record so far is 6 months for logf iles, before the flash became unusable.
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