Here's how burning fat works when in terms of time.
Your body has 3 primary energy producing systems. These systems are what "burn calories". Actually, what you're "burning" are the 3 energy producing nutrients in the body: carbohydrates, fats, and proteins.
The first of these systems is the phosphagen system, which runs on (and therefore "burns") carbohydrates. You only primarily utilize this sytem for the first few minutes of any intense workout - think of it as your spark plug and starter.
The second system, which is utilized once the phosphagen system is up and going is the glycogen system. This one is the second shortest, but still lasts you for 20 - 30 minutes of energy use. This one also breaks down carbohydrates for energy.
It's not until the third system, the oxidative system, that your body begins to burn fat. You use this system in two situations:
1. During day-to-day tasks (i.e., extremely low intensity energy usage)
2. After the phosphagen and glycogen systems have been tapped.
So, doing very, very light activity you are burning fat. However, since they require so little energy, you're burning almost no fat whatsoever (ca. 150 calories an hour). However with option 2, by doing heavy activity to exhaust the first two energy-producing systems of the body, you enter the fat-burning energy system (oxidative system) with high caloric demands, meaning that during and after your workout, you'll continue to burn lots of fat.
Your body can be "shocked" into using the oxidative system prematurely with workouts like high intensity interval training (see the HIIT subforum here), so that you can get a great fat-burning workout in just about 15 - 20 minutes, but you don't want to necessarily do HIIT every day. On different days, train for intensity - how hard you workout - and endurance - how long you workout at a moderate intensity - and you'll lsee faster results as long as you eat right and get plenty of sleep.