Beginner: Sprint at 6.0 mph for 30 seconds followed by 1 min break on the side rails.
Intermediate: Sprint at 7.5-8.2 mph for 30 seconds followed by 30 seconds break on the side rails.
Advanced: Sprint at 8.5-9.2 mph for 30 seconds followed by 30-45 seconds break on the side rails.
Total workout time is 30 minutes for all levels. *If need be work your way from 15 to 30. I also wouldn't recommend doing more than 30 minutes, because of the intensity.
Give it a try and let me know what you think!