Keeping backups until storage runs short (deja-dup style) with command line tools?

[Johannes Graumann <johannes_graumann@web.de>]

Hi,

deja-dup has an option to keep backups forever or until storage on the drive 
backed up to runs short (at which point it starts deleting old backups). 
Does someone have any pointers on how to copy that behavior using duplicity 
and/or other CLI tools?

Cheers, Joh