Short duration (16-20 minutes), high intensity cardio is a key element in the fat burning equation. A.K.A. Max-OT cardio.
Max-OT style cardio will have a bigger elevating effect on your metabolism and keep it elevated longer after exercise compared to long duration moderate intensity cardio. Plus Max-OT style cardio has less of any negative impact on the muscle building process.
Why 16 Minutes?
There is noting magic about the number 16. Your sessions could fall anywhere between 15 and 20 minutes. We settled on 16 because that seems to be a good time frame where you can keep maximum intensity and focus. The closer you get to 20 minutes the harder it is to maintain that high level of output. That's not to say 15 or 17 minutes wouldn't accomplish the task so don't over-analyze that number. The idea is to keep it between 15 and 20 and work your tail off every single minute!
How do I gauge intensity?
The best way I can describe it is you need to be "huffing and puffing" throughout the session and you need to be working out out of your comfort zone.
One tangible way to gauge this is to set distance goals and try to beat it. For example, if you travel 4.2 miles in 16 minutes on a recumbent bike then your next session you try to beat 4.2 miles with all settings being equal. Working to exceed your distance every time will quickly ensure you are working at the level necessary for maximum results.
If fat loss is a primary goal then I'd say you should be doing Max-OT style cardio 5-6 days a week.
If you are in a maximum muscle building phase and looking to maintain condition then I think 2-3 days a week is good.
What makes you so sure?
I was just like all of you and early in my career I did the customary long duration/moderate intensity cardio because that is what the magazines said to do. I achieved good results doing more traditional style cardio, in fact, I won many contests doing cardio that way. However, I always had a hard time getting that razor sharp condition until I implemented Max-OT style cardio. Results went from good to GREAT!
Max-OT style cardio was one of the biggest factors in the outstanding condition I achieved in 2003. It was without a doubt the greatest condition of my career and that is why I am so confident in Max-OT style cardio.