Every hour. Could do it more frequently if needed.

It depends on how resource intensive the backup process is.

Consider an 800GB Immich instance.

Using Duplicity or rsync takes 1 hour per backup. 99% of the time is spent in traversing the directory structure and checking which files have changed. 1% is spent into transferring the difference to the backup. Any backup system that operates on top of the file system would take this much. In addition, unless you're using something that can take snapshots of the filesystem, you have to stop Immich during the backup process in order to prevent backing up an invalid app state.

Using ZFS send on the other hand (with syncoid) takes less than 5 seconds to discover the differences and the rest of the time is spent on the data transfer, at 100MB/s in my case. Since ZFS send is based on snapshots, I don't have to stop the service either.

When I used Duplicity to backup, I would backup once week because the backup process was long and heavy on the disk array. Since I switched to ZFS send, I do it once an hour because there's almost no visible impact.

I'm now in the process of migrating my laptop to ZFS on root in order to be able to utilize ZFS send for regular full system backups. If successful, eventually I'll move all my machines to ZFS on root.