Tape is a fairly durable medium, as computer media go. For example, Exabyte says that its Mammoth tapes will last for 30 years. That is, you should be able to recover the data on the tapes 30 years after it was written.
However the actual effective life of tapes depends on a number of things. For one thing, Exabyte points out that the 30-year figure is for archival use. That is the tape is written once and put away. Tapes used for backup and regularly written and erased have a much shorter life, usually determined by the number of times the tape is used.
How the tape is stored also has a major impact on how long it lasts. Tapes should be removed from the drive as soon as they are used and put in a proper container. They should be stored in an area with more-or-less constant temperature and humidity. Generally, conditions which are comfortable for you are suitable for tapes. Exabyte recommends letting the tape rest for at least 24 hours if it moved to an area with different temperature and humidity.
Exabyte has a white paper on caring for tapes at http://www.exabyte.com/support/online/documentation/whitepapers/mediaguide.pdf. While the information is aimed at users of Exabyte's Mammoth tapes, much of the information is applicable to all kinds of tapes.
Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.