There are no hard and fast rules about the lifespans of backup tapes. Ideally you want to get plenty of usage out of the tapes (as they’re expensive), but you don’t want to risk your data.
Backup Exec has a Media Errors report which can be used to see how many errors have been tracked on each piece of media. It’s normal to have some errors, but when you see a tape with a significantly larger number of errors than the others, it might be a good idea to stop using it.
You can look at the tape’s properties in the media view in BE too, which tells you the number of hard (unrecoverable) and soft (recoverable) errors for each tape. Tapes with hard errors are bad, and should probably be retired.