Where I work we keep every single backup, we keep an immediate copy here in the building of the previous day's data, then backup the previous backup to another server somewhere else in the country. There at the other location I think we operate the father father son theory in which you hold 3 copies of that data. Depending on which DB and how important the data is on that DB, we have more/less backups than what i've just said, but thats the typical setup we have here for standard importance DBs