Misconceptions

The idea that you start out burning calories from carbohydrates (or glycogen) during a cardio workout, and then move onto burning fat once you've used up all your glycogen reserves is untrue. You are always burning both fats and carbohydrates at the same time, merely in different proportions depending upon the activity.

Time Frame

The body begins to produce elevated levels of fat burning enzymes 12 minutes into your cardio workout. So even though your body is actually burning fat all the time, your body will begin to burn fat at a higher rate 12 minutes into an aerobic activity.

Goal

In order to maximize your fat burn, go about your cardio workout in a way that will maximize your calorie burn because the more overall calories you burn, the more fat calories you will also burn. This means working out harder and longer which can be accomplished through interval training.

Method

Interval training involves periodically increasing the intensity of your workout, to the point of making yourself winded, for brief 30 to 60 second bursts. After the burst, resume the normal pace of your workout.

Recommended Amount

It's generally recommended that you get at least 30 minutes of daily aerobic exercise for fitness purposes. When exercising for the purpose of losing weight, the American Council on Exercise recommends a cardio workout of no less than 45 minutes.

Source:

UNM: Fat Facts

Science Daily: Interval Training

ACE: Fit Facts

More Information:

Penn State: Fitness FAQS

MSN: Triggering Your Body to Burn Fat

Ask Alice: Does exercising on an empty stomach maximize fat burning?