It takes place immediately after you create a scheduled backup, then after specified interval. That means if you added daily backup at 8am, then first will happen immediately, next at 8am tomorrow.
Of course as with all the schedules within WordPress, you need some real traffic that will trigger cron jobs behind the scenes. Any external uptime monitoring service should work well for this purpose, eg https://uptimerobot.com/
Please let me know if that answers your question,
WBR,
Alex