I used to do a ton of data-retrievals during litigation - pulling Iron Mountain LTO tapes back by the container (24 per container) - and restoring email and other data for the lawyers to comb through. We had a smallish 48-tape library, and this would have been 2003 - so old school LTO-1 I'm guessing (?).

Data was only 3-4 years old at most, but there was never a recovery failure - at least one that was visible during the restore process.

Oh - important data-point - we never re-used tapes. It was one-and-done-and-off-to-iron-mountain. (That eventually would have changed - but we were only 4 years into our tape-rotation when I left)

Anecdote of 1, but it was over probably 10+ restores of data. Not certain how many tapes were actually read - we would just jam them in 24 at a time and let the index figure out which ones to restore from.

The caution to validate backups by doing constant restore + validation is a good one though. Nobody cares about backups, it's the restores they want.