Although media tape can be kept for a very long time, just like revaroo mentioned, we don't usually keep it that long.
10 years or less is our practice here, there is a good reason for this:
For example, many years ago you may still be using DLT-type of tapes for backup, and you keep those tapes in a safe place - only retrieve it and put it back into library when you need to restore. You may recycle some of those for reuse. Regardless, quite possibly you are going to have new technology coming in and replace your backup operation (with disk, faster tape drive etc.), and usually people just leave/resell the old devices - and eventually those old devices will be out of sight.
Many may take this for granted that newer tape drive can still read the old media, but that is not always true. So before you get rid of your old devices, retrieve all your offsite old media and re-import them to a new media type (e.g. LTO5). Of course this will take time, but it will significantly drive down your cost of offsite storage (less media with bigger storage capacity) and the point of failure when you really need to restore some very old data (and you can't find old devices to do that).
