Alternate the intensities and length to keep the body guessing. I would do a minimun of 20 minutes at a high intenisty and a minimum of 45-60 at a lower intenisty. Also alternate the exercise. (Cycling, stairclimbing, running, rowing, swimming, walking.)

I have been taught that the best time to do cardio is upon awaking. Your body has been deprived of nutrients all night and will utilize fat as a fuel sooner.