Sensible database backup retention policy?
There are three questions that drive answers here:


How much space is the business willing to pay for to store backups?
Are there legal requirements to store backups for a certain period of time?
How often are you checking for corruption (running DBCC CHECKDB)?


For larger amounts of data, the first question usually answers the question straight away. If there are legal requirements, that can also answer things pretty quickly. 

If neither of those impose a limit, then the third question is a good starting place. 

Say you run DBCC CHECKDB and find corruption -- you're going to want a full backup to use in case you can't fix things in-place. If you're running it once a week, then keeping two weeks worth of backups is probably a good idea.

If you're not running CHECKDB, well, go do that instead of this.