Hi, as far as my trainer told me, you begin to burn body fat after 40 mins of any cardio, does not matter, but the machines I love the most is the treadmill (with a high incline and going a fair speed) and the eliptical trainer and there are different types of cross trainers but those are good as well, I find bikes are no fun and they only burn half the calories compared to most other machines, and eventually I would through a stairmaster machine in once a week, and I only started out 3 mins on that, now I am up to 8 or 10. They are not easy and not fun, but its a nice challenge!! good luck, hope this helps!