Both my nutritionist and trainer suggested for cardio that I keep my heartrate around 140-150 for 30 minutes. I do that about three times a week after my workout and I've been doing well, though only into my third week of my new routine. Then again, not sure how differently such a cardio workout affects male and female. For me, it doesn't take much, 6 incline at about 3.7-3.8 km/hr. Heh, I'm not used to all this though, I really hate cardio, but I push myself to finish and I feel great afterwards.