I can say from personal experience that backups are far more likely to happen regularly if your backup software runs automatically on a schedule. And let me be quite clear: regular backups are the only kind that matter. I think it's fair to state this as a corollary to Murphy's Law: "The likelihood of suffering data loss increases in direct proportion to the elapsed time since your last backup." In other words, if you're performing all your backups manually, the one day you forget (or run out of time) will be the day something goes wrong.
One consideration in choosing a backup schedule is media management. For example, if you're backing up to a recordable DVD, you must be prepared to insert a blank disc whenever the schedule runs. Swapping media can be an intrusion into your normal routine (especially if that routine involves the frequent use of other discs in the drive you use for backups). On the other hand, if you schedule backups to run when you're not around, you must always think ahead and make sure the drive has the necessary media ready. If, on the other hand, you're backing up to a hard disk or network device that can stay connected all the time, this problem occurs less frequently, if at all.
Depending on the speed of your computer, which software you use, and how you configure it, you may find that your computer slows down significantly while backups are running. This could be an argument for scheduling backups for when you're not using the machine. However, if you do not leave your computer on all the time, you will need to take special care to ensure that it's on and ready when the backups are scheduled to run (see the sidebar Power Management and Backups, page 171, for more information).
How often should you back up your computer? And if you're making both duplicates and archives, how often should you update each?
No single answer is right for everyone, but as a starting point, my rule of thumb is that duplicates should be updated at least as frequently as major changes to your system (such as installing Mac OS X updates or new versions of applications), and archives should be updated every day you make minor changes (receiving email, modifying text documents, and so on). Thus, if you use your computer heavily every day, and often install new or updated software, you might opt for weekly updates of your duplicates and daily updates of your archives. On the other hand, if you use your computer only occasionally, the schedule could become once a month for duplicates and once or twice a week for archives. Under no circumstances do I suggest backing up less frequently than once a month or more frequently than twice a daythe risk is too high in the former case and the aggravation too great in the latter.
Always update your duplicate just before installing system software updates. That way, if the new version of the software contains any serious problems, you can easily roll back your system to its previous state.
There may be some cases in which you could not afford to lose even a half day's work in the event of a serious problem, making twice-daily archives seem inadequate. If you're working on an important document, there's nothing wrong with copying it to another volume once per hour or as often as you feel it's necessaryor scheduling your backup software or a synchronization utility to do so for you. But updating an entire archive that frequently is likely to slow your work.
For more specifics about configuring your backup software to run on a schedule, read Automate Your Backups, page 170.