LAM, the question I ask myself is, "How many RESTOREs am I willing to do? Really - how many?"
I've found myself wandering thru 4 or 5 backups (a BACKUP folder on a hard-drive) and I'd find myself numb, forgetting what I was originally looking for and often stumbling across something else).
So, for me, "5" is about my limit. That means if I keep "6", I've exceeded my probable interest level.
But I usually burn a DVD to make a disk-backup as well, and set that aside 'forever and ever', wiping out those hard-disk copies eventually.
I think the best answer remains, "How many RESTOREs am I willing to do?" Backups are nuthin'. Restores are where it's at.