According to the Mayo Clinic, a 240-pound person burns 796 calories during each hour of high impact aerobics, which means that it takes 4.4 hours of such exercise to burn 3,500 calories. One pound of fat is equivalent to 3,500 calories.
The Mayo Clinic notes that the number of calories a person burns in an hour of exercise depends on the individual's weight and the type of exercise. A 160-pound person burns 533 calories in an hour of high impact aerobics, needing 6.5 hours of such exercise to burn 3,500 calories, two more hours than a 240-pound person needs. A 240-pound person takes just over eight hours of leisurely bicycling to burn 3,500 calories, while a 160-pound person takes almost 12 hours.