The optimal rest duration between sets is 45 seconds to 2 minutes to improve muscle endurance as rapidly as possible. Traditional endurance training (light-moderate weight, 15-20 repetitions) derives a large portion of its energy from aerobic metabolism. In contrast, high-intensity interval training (heavy weights, 8-12 reps per set) relies primarily on anaerobic metabolism for energy production. The more frequently you perform an exercise, the more muscle fibers you will build. For example, if you performed a leg extension exercise three times a week, your muscles would experience significant growth during each workout and during the days in between. Consecutive days of training are essential for achieving maximum results from your gym time. Resting your body properly allows your muscles to recover and complete their contraction cycle correctly without injury. You should rest your body adequately after every workout. As a rule of thumb, you should allow at least 48 hours between workouts to let your body have time to repair any damage done by the exercises. This means you shouldn't do two consecutive workouts before at least a day has passed. However, if you feel you can handle it, you can repeat a session after only six hours has passed. Make sure you're resting enough otherwise you might be putting yourself at risk of injury.