The American College of Sports Medicine has exercise guidelines for healthy adults. The guidelines are based on current research to help people maintain good health.
Aerobic exercise - Aerobic exercise has an additive effect. Cardiovascular benefits gained in three 10 minute exercise bouts are almost the same as those from one 30 minute exercise bout. The recommendation is that people should perform aerobic exercise at least 5 days a week at 50%-85% of maximum heart rate. Your maximum heart rate is 220 minus your age. Unfit people can exercise at 40% of their maximum heart rate. Exercise should be done at least 30 minutes. Ten minute bouts of exercise can be accumulated throughout the day.
Resistance training - Several studies show that one set of 8-15 repetitions provides nearly the same strength and endurance increases as three sets. People younger than 50 should work the major muscle groups two or three days a week with weight loads that allow 8-12 repetitions. Those older than 50 should resistance train two or three days a week with weight loads that allow 10-15 repetitions.
These recommendations should be considered a minimum amount of exercise that should be performed every week. More exercise is better.