Question : Problem: Backup Tape Life

How long should you use backup tapes before you should think about replacing them?

Answer : Problem: Backup Tape Life

Well...
It depends how often do you use it (1), and how much is it old (2), where first one has more impact to durability.
You'll notice when drive heads get dirty too often, that is a good sign that tapes are old (magnetic stuff starts to fall off the tape-sorry for my bad english).
I use DLT/LTO/DDS tapes every day for 2 or so years... Everyday use.