Data must be restorable to a current state for core business continuity, or to a point-in-time for audit purposes. In fact, some regulatory requirements may mandate specific retention schedules other than what's outlined below, though these are good general guidelines:

The Three-Week Rule

You should be able to restore a version of a file from any workday for the past three weeks. To do so, you must back up core data daily, and maintain at least three full weeks of version history on unique media. No tape should be overwritten for three weeks, including daily incrementals!

Weekly Full Backup

Weekly backups should be performed on operating system data, B/R software data, content data, configuration files, registry files and application software data. Weekly data should not be overwritten for at least one month. This set of data should be considered for duplicates.

Monthly Full Backup

Similar to the weekly full backup, this monthly activity needs to include all data content and files and not be overwritten for at least one year. This data should be duplicated as well.

Annual Full Backup

In this case, all data and files are vaulted off-site immediately, where they are typically retained many years as specified by the company's regulatory demands. This data must be duplicated.