How to: Set the Backup Retention Duration (SQL Server Management Studio)
Use the media retention option to provide a system-wide default for the length of time to retain each backup set. The media retention option helps protect backups from being overwritten until the specified number of days has elapsed. When you set media retention, you do not have to specify the length of time to retain system backups each time you perform a backup. The default is 0 days. If you use the backup medium before the set number of days has passed, Microsoft SQL Server issues a warning message. SQL Server does not issue a warning unless you change the default.
To set the backup retention duration
In Object Explorer, right-click a server and select Properties.
Click the Database settings node.
Under Backup/Restore, in the Default backup media retention box, type or select a value from 0 through 365 to set the number of days the backup medium will be retained after a database or transaction log backup.
The default value is 0 days.