Fitness Level gains are determined by Frequency, Intensity and Duration of the Aerobic exercise. Each session (duration) should last from 20 to 60 minutes and be performed 3 to 5 days per week (frequency) at an intensity level measured by heart rate (60% - 90%) according to the American College of Sports Medicine (ACSM).
During the first 15 minutes of aerobic activity, glycogen or sugar within the muscles is used for energy. Fat metabolism for energy doesn't occur until about 15 to 20 minutes after beginning aerobic activity. This is why it's important that aerobic duration be at least 30 minutes. Aerobic sessions greater than 1 hour continue to burn fat but at not the same rate as during the first hour.
Additionally, sessions greater than 1 hour increase the risk of injury due to fatigue. Increasing aerobic frequency (greater than 5 times per week) does not give the body a chance to fully recover and can even reduce the body's capability to defend itself against illness. It is important to listen to what your body is trying to tell you. Rest, adequate sleep, and proper diet all become more critical when demands are placed on our bodies above the normal everyday physical stress.