Tape backup size versus file retention time
Backup to tape is great for long term retention, but for recovering from mistakes, we used backup to disk.
Depending on your total storage, this can be a cheap way of keeping at least one copy of every file cheaply and quickly.
Say you have a total of 4TB of storage to backup. It doesn't cost much to build a 12TB backup volume and backup to there. Then keep your tapes for long term archival, doing full backups and shipping them off site or into a fire proof safe, etc.
Typically, if you care about recovering your data after a disaster in an acceptable amount of time, full backups are done regularly (weekly, bi-weekly, whatever) and incrementals or differentials are done daily. Differentials will allow you do do a full recovery in three steps:
Restore the latest full (which will only be a week or two old).
Restore the latest differential.
Go have a snack.
Incrementals will require you to restore every single tape since the last full backup was taken. This is not a good thing if you just have never-ending incrementals.
Also, keep in mind that backups stored on-site are subject to destruction in a disaster just like your servers are (flood, fire, angry ex-employee). Send them off-site. Send them off-site regularly. Tape rotations are quite common.
I have about 200 tapes in my rotation right now. We use about 12 for a full every weekend, and between 3-5 per day for the differentials. We have 5 tape "sequences" that are basically a week long. Every day a courier picks up the day's tapes and brings back a box from 5 weeks ago. The only tapes that we keep on hand are what we need for the week's backups. Everything else is off-site in a hardened facility, in a fireproof case. This is the only way to do it if you want to recover from a disaster.
When the tapes return from the DR facility and are fed to the hungry tape robot, they're added to the scratch pool and are overwritten. At most, I will only ever lose a day's worth of data and I can go back as far as 5 weeks if necessary. There are other policies for certain things that legally need longer retention, but that's another story and they're the exception not the rule.
Of course disk-to-disk is good too for quick restores, but offite archives are a must for DR.
tl;dr - Most people don't just use the same few tapes over and over. The best way to do things if you care about archival recovery in the event of a disaster is to have multiple tape sequences that are cycled off-site, and just overwrite the oldest when they come back unless you have a legal reason not to.