As NAND is a relatively new technology with the manufacturing process changing regularly, it is difficult to state an actual lifespan. As far as I'm aware of, most NAND is rated in write cycles rather than a given time. However, with SSDs having MTBF ratings of over 1 million hours (i.e. >100 years), this seems to indicate that NAND can potentially hold data for over 100 years, at least the NAND used in solid state drives.
Since I first started using SD cards back around 2004, I have only had a single failure, which was from a generic non-name brand SD card, unlike the countless recordable CDs and DVDs that have been binned over the years. I have a few cards that are in regular use over two years and still working well and others I retired due to their low capacity, but which also still work fine. My parents treat SD cards like film, where once they fill up a card, they use a new card, so between this, the pictures on their PC and on external media, their SD cards effectively act as another backup.
For any precious pictures, I recommend keeping at least two backups (e.g. on the original SD cards and an external HDD) and another online if you can afford it (in case of fire, flood, etc.)