The American College of Sports Medicine has stated in its position statement that Cardio (aerobic activity), be performed on most days of the week, for at least 20 minutes per day. For anaerobic activity, such as weight lifting, they state that you need only perform one set of 10 repetitions, per bodypart (ex: chest, back, shoulders, triceps, biceps, abdominals, calves, thighs), two times per week to maintain lean muscle and strength.