This article is from the Aerobics Fitness FAQ, by firstname.lastname@example.org (Robles) with numerous contributions by others.
You are _always_ burning fat. There is no magic on/off switch for
"fat burning", or any other system in the body. Your body gets its
energy from several sources all the time; the proportions change
depending on the intensity and duration of the activity, but stored
body fat is always one of them.
Stored body fat is utilized more for low- to moderate-intensity,
long-duration activity; this could be where the confusion about
needing to exercise for x number of minutes arises. During the
first several minutes of exercise your body gets started by
tapping primarily (not exclusively!) its more immediate energy
sources, like glycogen in your muscles. These sources cannot keep
up with the continued demand for energy, so your body gradually
taps into stored body fat as well to continue at that intensity.
As an analogy, think of your body as always carrying around a cord
of firewood and a small bottle of jet fuel. The firewood is your
stored body fat, the jet fuel is the glycogen in your muscles. When
you need to suddenly dash for the bus, you use the jet fuel. It
won't get you far because you don't have much, but you can get
there very fast. When you need to go on a long day hike, you use
the firewood--a long, slow burn that can last for hours, and you
have plenty of it. But you always have to use a few drops of jet
fuel to get going while you kindle the firewood, and to keep the
flame bright. And you always have to use the firewood, even if
only as a pilot light.
From this analogy it should be easy to see that it's not possible
to work out ensuring that 100% of your energy is coming from fat--
you couldn't get started, or once started couldn't perform at more
than a "slow smolder" intensity. In fact, many people report
feeling very sluggish if they try to work out in the morning on an
empty stomach, while simply eating a simple high-carbohydrate snack
an hour before yields a much better performance. It takes energy
to make energy!
It may at first come as a surprise to learn that the time when your
body is getting its highest _percentage_ of energy from fat is when
you're asleep! But consider that when you're asleep, your body has
no need for bursts of high intensity activity, so those energy
systems are quiet. Obviously, sleeping is not a good activity for
losing weight, because your total energy requirements are quite low
then. This should show you that the _rate_ at which you burn
fat/calories is not as important as the _total_ that you burn on a
daily basis. In other words, the minute-to-minute fluctuations in
the proportions of fat vs. carbohydrate used by your body are not
linked to long-term weight managment.