Well I do at least 2 and sometimes more. I usually do Imax 3 (my favorite) and plyo X from P90X. I also go to the rec center and do 40 minutes HIIT on the elliptical trainer, and then 20 min HIIT on the stepper once or twice a week. This is real calorie burner!
If your a runner, you could add intervals into your running.
I have hypothyroidism and cardio is the only way I can get myself and my metabolism moving everyday. If I wasn't hypo or didn't have weight to lose, I would probably cut down to 1 or 2 HIIT workouts per week.
Diana