I believe it automatically runs at midnight, but I would rather it run during an off-peak internet period to save my quota.
How can I change the time it runs?
That feature is on the wishlist, for now you can try setting the schedule to monthly in the preferences and setup your own schedule using crontab and deja-dup --backup
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You can do this by disabling auto updates in Deja-Dup (Backups) and then running a cron job at a specific time.
To run daily at 2:00 AM add the following to crontab
0 2 * * * deja-dup --backup