What is your goal?
If you are looking to do cardio to keep your cardio vascular system healthy, you should do at least 30 minutes a day of moderate intensity cardio (zone 3 or about 60%-70% Max Heart Rate).
Are you looking to lose fat. Longer intervals of moderate intensity. Are you looking to improve your speed and endurance-
HIIT (short interval of high intensity).
But you may want to check out the
HIIT forum of this website. I believe that
HIIT sessions require days of recovery for optimum effect, but I'm not expert.