I average a fully loaded NAS every 8 years apparently. I don't really expand their storage, as by this time the network and software is out of date, so I buy a new one and make the current the backup to the new, and then dispose of the one that was the old backup.

It's interesting, the upgrade interval has held for 20 years already and so it becomes easy to understand the amortisation, 16 years of use, but really only 8 as the primary and 8 as the backup.