Backup critical information when you feel concerned about losing it.
This strategy is used predominantly by enterprises that do not have a standard approach and by individuals who may wish to save their own work but where that work has little impact on the enterprise. Examples include computers operated by students and professors at universities, personal computers used by individuals at home, and research computers used for experiments but not part of operations.
Backup everything periodically.
This generally only applies to systems with medium or high consequences of failure and in cases where these systems do not have standard configurations or other methods for rapid recreation. This is common in systems designed for special purposes, research and development systems, and for enterprises in which there are relatively small numbers of individuals working on independent projects. The most common approach is a monthly compete backup with annual permanent storage, weekly incremental backups kept for a month, and daily incremental backups kept for a week. As disk-to-disk backup has replaced tape and other media backup, backups to file servers has become increasingly dominant.
Create and maintain different backup processes and schedules for different information based on its criticality.
In most enterprises, there are enough differences between systems that a mixed strategy is applied using many of these approaches depending on the specifics of the systems involved.
Continuously and automatically backup everything all the time.
In transaction systems or increasingly in resilient infrastructure systems and to a lesser extent in systems with built-in incremental backup systems, like Apple's "Time Machine", backups are done either immediately or with frequencies high enough that almost everything ever done is recorded.