For a hosted property, the answer is NONE.
A property managing their own servers and workstations must maintain all the backups needed to restore the system to full operation after any type of hardware failure or disaster such as a fire or flood.
To achieve this level of recovery consider doing the following:
- Have an image backup of each server
- Update the backup if any system parameters are changed or the system is upgraded
- Keep a copy of these backups in a separate safe location
- Do a full database backup at least weekly
- Do an incremental database backup at least daily and more frequently is preferred
- Copies of the database backups should also be stored off site
Avail also recommends using some level of RAID (redundant array of independent disks) to provide real time recovery from hard drive failure.