The frequency of backups will depend on the type of data to be backed up.
For example, if you are running a network environment with critical data stored on your servers, then even nightly backups may not be enough.
On the other hand, if you are backing up the data on your home computer then you may need to factor in a few things, such as:
The amount of time you spend on the computer.
How often and by how much the data on the computer changes.
If it is just some music, e-mails and family photos, then weekly, monthly or even less frequent backups may suffice. However, if you happen to be in the middle of a tax audit, more frequent backups may be necessary.
Explain how often the user should back up and why (there may be different frequencies for different types of data).
Explain that backups can be scheduled, and link to topics which explain how scheduling is done.
Will be a short topic.