Was looking at ways to schedule and found Cron on wikipedia.
https://en.wikipedia.org/wiki/Cron
This would be a nice way to shedule backup jobs.
The input could be with the use of a GUI, to make it easy for the general users, but it should be easy for Duplicati to keep a table with Cron line + backup job and just execute the backup when the cron time is triggered.
This Cron thing is really well thought out 