"One study in the Journal of the International Society of Sports Nutrition reported that male subjects following a 6-week HIIT program (done for 15 minutes per day at a ratio of 2:1 for exercise-to-rest, 3 days per week)" It depends what your useing it for though if its your only workout then 3-5 times a week is plenty if its replacing cardio in your regular w/o and you still go to the gym then 2-3 days a week would be lots as you could use it as an active rest day or a mourning fat burning session before your w/o. don't do it everyday or your will likely go into an 'over trained' state. Chris TheBoss Smith.

While he doesn't explain, I'm sure you could do a little more research to figure out how it works into your own workouts. It's pending what your goals may be. I think Chris "TheBoss" Smith put it very well. I'm starting over Jamie Eason's Live Fit Trainer, definitely going to do this routine 2x a week as my active rest days.