Many people still don't realize this, but flash memory wears out the more you delete and write files to it. How quickly it wears out will depend on the specific flash drive. Like anything, there is a wide range of quality in currently available flash memory-based drives. You can make a rough correlation with the cost/quality of the device (what you pay is what you get), but even that tends to change over time. Depending on usage and quality it could be talking as short as months or as long as several years. In extreme cases of misuse it could lasting as little as a few weeks.