10-20 years in my environment (60-95% humidity if not controlled) is what I expected. And that's what I've been getting from older left-in-storage drives since 2000 (all of them still ok).

While I'd hope professional backup tape is a little better than high end early 1990s VHS tapes, the 10-20 years that beagle3 expects is also in line with what is generally expected out of modern backup tape. Granted, we are talking about archival storage, and it is still used for that, but after disks got big enough that tapes were seldom used for more than backups the pattern is to continually refresh them with multiple backups from disk.

It may be better, or worse. But temp matters a lot - several data centers I've helped in are often left to get to almost 80 degrees - I can see that being a huge problem on their lifespan!

I know from experience that people are very likely to think "Tape?! It'll last forever!...right?" which is dangerous.