So was it a silly question, or does nobody know the answer?

Only you can determine what rate of backup is suitable to your needs.

Here at my place of work, it's done overnight, every day. A set of 5 tapes is used, one for each weekday. Periodically these daily copies are archived away and stored off-site. The tapes themselves are retired after so many uses so as to avoid failures due to wear. So in the case of a failure, we can generally guarantee to lose no more than 1 day's worth of traffic. If there's a serious incident (fire?) that renders the daily tapes unusable, we can fall back to the most recent off-site archive.

None of my own personal email is so important as to justify daily backups. You need to make your own choices. You might decide that only certain messages are critical, so you could look at filing them so as to make backing them up separately as quick and simple as possible.

Also make sure that whatever you do and however you do it, you can restore from it in a useful and meaningful way.