Calculating data retention rate of SLC NAND
You didn't link the device datasheet, but a typical SLC NAND will use 512 byte pages, giving a total of 524288 pages in a 256 MB device. You are writing 14 pages per day, so it will take 102 years to completely fill the device.
Since your design spec is only 15 years, you will never erase anything and could actually use memory that was not reprogrammable if such were available.