A week from now. Six months, perhaps. But it will happen, and as Murphy's Law states, it will happen at the worst possible moment. Now that we've agreed on the inevitability of such disturbing events, let's talk about preventative maintenance. Yet, like so many important tasks, we procrastinate it.
Backups can wait until tomorrow.
Backups can wait until next week. Backups can wait until it's too late. This is a subjective question, but I'll try to give an objective reply. It depends on how important your data is and how often the data is being modified. If you only use your computer on Sunday afternoon to check your mail and type a few Word documents, chances are you only need to back up those items once a week.