I see some characterization data for the internal flash memory in the datasheet. Taking EFM8LB1 as example, the typical and min write/erase cycles are 100k and 20k. What does this mean?
The flash endurance specification guarantees that each bit in the flash can be written and erased at least 20k times before bit errors can start to occur. Specifically, it refers to the number of times a bit can be transitioned from a '1' to a '0' (a write) and back to a '1' (an erase) before failure.
Note: Erasing (setting to '1') a bit that is already a '1' does not contribute to flash fatigue, so erasing a flash page only affects the bits that were '0' before the erase operation. In the same way, writing a flash byte only impacts bits that transition from '1' to '0'.