Backup tapes

Strange question, but……

Does anyone know how long an average backup tape should last? I have a friend who is using Imation’s MAGNUS 1.2 backup tapes and they are only lasting for about 75 uses. Is this normal? She swears that she has had them last for twice that in the past, but all recent tapes wear out after less than 100 uses.

Thanks in advance.
Sid