I have instructed DejaDup to keep one-week of backup files. Now it is passed more than a month and my backup storage is fed up with old backups. Is there a way to instruct DejaDup to delete old backups without reaching the storage limit? Can I detele them manually?
You can use the Dconf app from Ubuntu Store to modify setting at path org.gnome.DejaDup, key name delete-after. It's set to the number of days to keep backup files on backup location.
Or from terminal. For example, to set it to 60 days from the command line, run:
gsettings set org.gnome.DejaDup delete-after 60
Deja-dup does not yet supply a way of removing old backups, you should also not delete some of the files, that will leave probably your backups without a start file and renders them invalid. Remove them all and start over is an option but thats not what you want I think.
Deja-dup keeps backups for the specified time or until the backup space is full, it will them manage your backups accordingly, a solution for your problem might just be enable quotas for the backup drive and don't let it take all the space available or change the backup frequency, once a week if you edit many files or are always copying / moving files will leave you with a very large backup image.
The answer of https://askubuntu.com/a/94288/676490 for a local backup:
duplicity remove-all-but-n-full 1 --force file:///home/username/deja-dup