So, for example, it takes 52 passes for a DLT 7000 to fill a tape, and another 52 passes to verify the backup. Assuming you don't do a restore (which is another 52 passes), you can back up to that tape about 5,000 times.
This is assuming the drive is writing correctly of course. In addition, most people don't take care of their equipment, so it is probably best to replace the tapes ahead of the recommended specs.
If you're doing weekly backups, you probably want to replace the tapes every two years. In the end, it's better to spend the money on new tapes than just as much money to do data recovery on one tape or risk losing information critical to the operation of your business.
Read Pierre Dorion's answer to this question.