That's a very good question. According to the manufacturer, each block of data should survive at least 100 000 write cycles. The manufacturer told us that they are using these type of chips for more than 5 years, and never had returned boards with dead NAND. The boards that we using for firmware testing have no issues after more than a year of daily wipe-out and re-flashing. On these boards, we flash the firmware about 10-20 times a day, some times more. And none of the boards died.