In my documentary I learned that you don't want to go over 20 minutes if you are trying to build more muscle. More time than that will tell your body to trim down the size of your body so it will work more efficiently. Have you ever seen a large muscular marathoner? No and you never will. Okay then why do those big guys in muscle mags say 40 to 100 minutes of cardio. Well they are on steroids and that changes the picture. If you want to build a natural body and you should, then don't do any of those workouts in muscle mags. They are designed for the "roid boyz and womenoids."
More...now that you understand the 20 minute rule I have to stress something here. In order for you to burn fat on the cardio machine or bike you must try to maximize your workout. What I mean is that you should try your hardest to best your distance over time every time you do cardio. If you biked or ran 4 miles in 20 minutes then you should try to best it the next time you do cardio in the same amount of time.
This will probably be the most challenging 20 minutes of your day. Eventually you will peak out and not beat your time. When this happens try another machine and best a time on that. Cardio is cardio no matter the type of machine.
Use this rule 20 minutes if you want to grow more muscular and spend less time in the gym.