How long to lose weight by exercise?

A pound of fat contains roughly 3200 calories, so whatever amount of time it takes at a certain intensity to burn 3200 calories is more or less how long it will take to burn a pound of fat. I say more or less because the majority of energy used during exercise comes from glycogen (carbohydrates) instead of fat. Assuming this glycogen were not immediately replaced, your body would have to get that energy from fat after exercise.