What is the expected DLT tape longevity ?

From Backup Central

Tape media longevity can be broken down in to 2 sections:

1) Shelf life; What is the estimated time a tape can retain the data that is stored on it.

2) Usage life; How many times can a tape be accessed before it becomes unreliable.

Shelf life: The DLT tape media shelf life is more than 30 years. The environment is specified as 18°C to 28°C (68°F to 82°F) with 40% to 60% relative humidity (non-condensing). The cartridge should be stored in its protective case to keep out dust and contaminants.

Usage life: The DLT tape cartridges are specified at one million passes. In laboratory tests, the media has been usable even after very high numbers of passes and the media actually seems to improve with use. At one million passes the tape should continue to function normally, unless it is being used in an environment with heat, humidity or contamination levels beyond those recommended. A pass refers to a single movement of a section of the tape media across the head assembly. The number of passes may be determined by the number of times the cartridge is read or written "end to end".

In short: You should be probably more worried regarding if you have still the equipment available to read the tapes as opposed to if your tape is still readable.