The damage to the memory cells of flash memories depends on the number of program and erase cycles that have taken place and on the speed at which these cycles take place.

With the typical lifetime of a flash memory medium, the cycles are mostly evenly distributed over a period of at least three years. The period between two cycles is then sufficient for the charges in the
tunnel oxide to detrap, and the flash reaches its specified number of cycles. In the case of an atypically short cycle time (especially in the case of qualification tests), there must be sufficient time between the cycles for
the cells to recover and / or the temperature must be increased in order to achieve the specified service life.

Detailed description about NAND flash endurance testing