Hi,
since PBS doesn't pull in backups, this depends on the client. You just need to set up a monthly cron job or systemd timer that runs your proxmox-backup-client
command.
In case you're talking about integration in PVE, it's currently not possible to set up a schedule that's less than weekly without manual editing. But we are working on it. In the meantime you'll either have to edit the schedule for an existing job in /etc/pve/vzdump.cron
or create your own vzdump
cron job.