Many people have asked "when do I start burning fat?' during their cardio workouts. I suggest - try to stay in your target heart rate for over 20 mins ... anything over 20 mins is fat burning time. :)

One of the most accurate means of determining target heart-rate is the Karvonen formula. This formula calculates a percentage of the heart-rate reserve, which is the difference between the resting heart-rate and the maximal heart-rate.

Heart-rate reserve = maximal heart-rate - resting heart-rate

Maximal Heart-rate is the highest rate a person can attain during exercise. While an electrocardiogram test would provide the most accurate MHR, for practical application an age-predicted heart-rate formula was developed.

Maximal heart-rate = 220 - age

This formula is based on the assumption that one's heart rate at birth is 220 and decreases by one every year. The accuracy of determining maximal heart-rate based on this formula can vary at any given age by + 10 beats per minute.

Resting Heart-rate is the rate at which your heart beats at full rest. It is recommended that this rate be taken before getting out of bed, counting the pulse for a full 60 seconds, 3 mornings in a row and averaging the counts.

Interval Training

Interval Training can help you improve cardiovascular fitness, increase speed, improve overall aerobic power, burn more calories, break-through a plateau, increase workout time, reach new exercise levels, expand your workout options and increase your workout threshold – just to name a few.

Plus, this training method has useful applications for beginners, intermediate exercisers and even conditioned athletes. There are two basic types of Interval Training. For the majority of exercisers (novices and intermediate) Fitness Interval Training methods are recommended. Athletes can choose a more advanced technique known as Performance Interval Training.

You ask what is Interval training?
It is basically short high/higher intensity exercise with alternating periods of recovery.

Example:
(30 min cardio on treadmill)
Warmup 5mins at 2.3 mph at 0% incline
Jog 2 mins at 5-6 mph at 0% incline
Recover 3 mins at 2.5 mph at 0% incline
Jog 4 mins at 5-6 mph at 0% incline
Recover 6 mins at 2.5 mph at 0% incline
Jog 3 mins at 5-6 mph at 0% incline
Recover 4 mins at 2.5 mph at 0% incline
Cool down at 2.0-2.3mph at 0% incline

Continuous training (jogging or walking at one speed for a duration of time) is very effective and should not be eliminated from your weekly workouts. However, it’s recommended that you include both Interval Training and continuous training sessions as part of your fitness regimen.

To begin, choose the type of cardio exercise: walking, jogging, swimming, biking, etc. Next determine your lower-intensity level. This is usually somewhere between 50-65% target heart rate. This will be your baseline, lower-level intensity. Then simply increase the intensity-level up to where you feel like you are working hard to very hard, but avoid reaching a level over 85% target heart rate. If monitoring your heart is not feasible, instead use the RPE (Rating of Percieved Exertion) scale where 1 is basically at rest and 10 is working extremely hard. For example, if you find that when you are exercising at a comfortable level you rank a 5, then bump up to a 7 for the higher-intensity intervals.

Where you take it is up to you!
Good luck and great fun!