How long does a USB last? I have a USB loaded with some pictures that’s been sitting on the shelf next to my speaker for a couple of years.
Hard drives will last longer as long as they are not running 24/7.
DVD media has a shelf life, too depending on where it is stored. It is susceptible to higher temperatures and UV. A dark cool place is best.
Any valuable backup must be checked refreshed periodically.
That depends entirely on use and conditions... Hoefully the new SSD drives (no moving parts) will solve some of that uncertainty...
However... Duplication is the answer. You should always have your critical data backed up in AT LEAST TWO places... TWO devices... And the more the merrier. Even a well-kept DVD or CD has a shelf life.
That is one of the reasons why I like the NAS + USB scenario... It is unlikely that you would lose BOTH devices at the same time (providing the USB is off-line and unplugged) and if one fails, one doesn't lose data... And prompt replacement of the failed device assures continued data security.
I have a programmer friend, and outside of my local backup, I have access to his local server for backup purposes by way of FTP (and he has access to mine). Since he is in Illinois and I am in Montana, barring the apocalypse, we are very backed up not only locally, but afar too, and all within a tightly controlled environment (like cloud, but not cloud...)
How are you defining USB?
If you mean a USB flash drive, the answer is virtually forever. Flash drives use silicon chips for storage and are not susceptible to magnetic interference.
If you mean a hard drive connected via USB, I wouldn't trust it more than five years. And hard drives are susceptible to magnetic interference like that coming from a loud speaker.
FWIW, I have seen flash drives go through the laundry (wash & dry) and still work.