Backup tape useable life and archiving

The other day one of my clients asked me how long they should keep using the same data tapes. My inital thoughts were until they start to have errors. But then it occured to me that you might want to stop using then well before then.

So my question is, how do you tell if a tape is degrading?

Does the age of the tape and the number of time it has been used degrade its ability to store the data?

What archive cycles do other members use or suggest using?

DO