How Hard Do I Need to Exercise to Burn Fat?

To answer this question we need to focus on two areas of the FIT principle (Frequency, Intensity, and Time of exercise). These two areas are intensity and time.
Let's look at 2 sample workouts performed by the same individual exercising on a stairclimber.

	SESSION #1	SESSION #2
Percentage of MHR	85%	65%
Exercise time	30 minutes	30 minutes
Calories expended during exercise	480	300
Percentage of calories from fat	50%	80%
Fat calories utilized	240	240

These numbers are used for the sole purpose of demonstrating how a higher intensity cardiovascular workout can achieve an equal or greater number of fat calories utilized as a lower intensity workout. Individuals caloric numbers will vary.So you can work out at a higher intensity and still burn an equal or greater number of fat calories as compared to a lower intensity workout. The following are added benefits of the higher intensity workout.

Larger number of total calories utilized

Metabolism remains elevated for a longer period of the time after exercise.

You gain a higher level of cardiovascular conditioning.Keep in mind that individuals who are ill or injured should start at a lower intensity level until they have regained their health.
Should Burning Fat Calories Be My Goal?
I think not. I believe that caloric expenditure should be our main goal in exercise when we are trying to lose fat weight. The reason is that 3,500 calories equals one pound of fat. Theoretically if we ate an extra 3,500 calories in a week, we would gain a pound. The same applies to eating 3,500 fewer calories than our body needs in a week - we would lose a pound. Instead of eating 3,500 fewer calories and depriving our bodies of the fiber and nutrients that we need, let's eat nutritionally dense food, make healthy food choices, and expend those calories in the form of exercise. Whether the majority of calories expended are from fat or glycogen, ultimately calories are calories when it comes to exercise.
While keeping in mind that you don't want to exercise at such a high level that you can only last a few minutes, remember that the longer you exercise the more calories you expend, and the higher your intensity, the more calories you expend.
If you are in good physical condition and you're trying to lose or maintain weight, and you feel like exercising at a higher heart rate, go ahead.