Note that the actual backup frequency and cycles will vary with the operating system, backup software employed, and specific application needs.
Normal minimum backup cycle
Most systems should be fully backed up on a weekly basis. For active systems with high-availability needs, these backups typically take place offshift, i.e., overnight, on Sundays or some other minimal usage time. The backup media (tapes or whatever) should typically be stored in some secure off-site location, such as a vault or secured space in another building.
Systems files should be backed up whenever they are changed by systems maintenance or administration. This may or may not be adequately covered by daily incremental backups depending upon the system and backup software.
Online databases
In the cases of production databases that are updated online, weekly backups should be supplemented with daily full saves in addition to any "transaction" or "journal" files kept by the database software. These backups are commonly stored on site to ensure the information can be quickly recovered when necessary (e.g., for rerunning batch jobs). An important rule of thumb is that backups and logging should be sufficient to avoid reentry of business transactions in circumstances short of a major catostrophe (in which case, we will likely have more inportant things to worry about anyway).
Multi-user Servers
[This applies to any server that has files that are under the direct control of multiple users, for example, Unix (e.g., Zoo), Novell (e.g., Pandora) and NT/2000 (e.g., BSAD servers).]
In addition to the weekly full saves, daily incremental backups are the norm. This means daily, overnight backups for new and changed files.
Archival Purposes
Archival backups should be stored in an off-site, "fire-proof" vault. Actual selection of which data should be archived is typically determined by law or needs of the data owner, not computing technologists.
Note that there is a cost to archiving information &emdash; for selecting, cataloging, indexing and physical storage of media. There is an even larger cost of accessing the information. Public records laws may necessitate plans to access archived information. The decision to archive information indefinitely should not be made lightly. Fire-proof, environmentally-controlled storage locations are not readily available and costly to build. Media technology continues to evolve rapidly. Devices capable of reading information out of old media may not exist in only a few years. Media storage reliability is unknown for many media types. [We already know that tapes as young as 5 years may be difficult or impossible to read.] Periodic copying of information onto contemporary media may be necessary of magnetic or optical storage. Fire-proof storage locations are not readily available and costly to build.
Summary:
1 year or more
1 year or more