We back up our sites nightly on-site and retain 4 weeks of nightly archives. We retain archives from further in the past with decreasing density. For example, we might have a backup from every other night 4-6 weeks old, every forth night from 6-8 weeks old, and so on. Backups are also immediately streamed off-site, using an encrypted circuit, to permit for disaster recover as well as to provide a backup to the backup. The off-site archive stores a complete copy of the on-site backup.
Backups are always immediately available, and restores – of individual files or complete machines – can be quickly performed.