I have turned on "automatic backups" using Déjà Dup:
How often will Déjà Dup make a backup now? Is it once a day? The help documentation doesn't say.
Silly, me, I missed this option under the "Schedule" tab:
You can set it to either daily or weekly.
If those two options are not what you want, you can set it to a specific number of days by modifying a Dconf setting. For example, to modify it to two weeks, run:
gsettings set org.gnome.DejaDup periodic-period 14
You can also edit this setting with dconf-editor, the setting is found in the tree path