On Xopero QNAP Appliance solution, the daily notifications are sent once a day at 3.40 A.M. You can check this by connecting to the QNAP by SSH and entering the command crontab -l and find /etc/init.d/XoperoServer.sh opero_report_sender (first are displayed minutes, hours later).
If it comes to Xopero Backup&Restore solution, you can find the task Xopero reportsender in Scheduler available on your operating system, the task resposible for daily notifications is called "Xopero reportsender" and by default it is being executed by trigger on 6:00 A.M., but you can easily edit/add the trigger.