I have heard that it takes a full 20 minutes of exercise for your body to begin burning fat. The fat burns after that time, not during the first 20 minutes. Is this ture because I have been using a bowflex for about a month now and my results have been disapointing. Could it be because i only do it for about 25 minutes per session. In any event, i'm going to do 20 minutes on a treadmill after it.

Nutrition is as important to results as the actual exercise. If you've been eating poorly, that's the reason for the disappointing results.

I have read the same thing. I do 45 minutes to an hour and get good results from that. Also a month isn't very long. It's best to compare every 3 months or so.

Losing fat is different from gaining muscle. You should be using strength increases to determine progress with the bowflex. If you want to lose fat, eat a healthy low calorie diet and do aerobic exercise 3x a week for 30 minutes or more. BTW, the more muscle mass you have, the more calories you will burn, but it can't do it alone.

I wouldn't consider a bowflex as a cardio exercise but I guess that depends on how you are using it. How long you need to go to burn fat depends. You want to reach your target heart rate and from then on you can burn fat. At least that's how I understand it to work. Typically I've heard cardio for 20 minutes (minimum) because it's assumed you can reach your target heart rate in 10 minutes and then burn fat for another 10 minutes. Again, this depends greatly on what kind of shape the person is in and how hard you are exercising. If you are in great shape and just walking it's going to take a very long time to reach a target heart rate.