Your Target Heart Rate, from 60 to 80 percent of the maximum your heart can beat, should be your goal 3 to 5 times a week for 20 to 60 minutes (excluding warm-ups) to maximize the health benefits of cardiovascular activity, including losing weight. People who exercise regularly do build up endurance, but they gain little additional value when their heart rate goes above 80 percent of their maximum. Besides the strain and injuries that can result, the heart is simply working too fast for any benefit, and your body cannot replenish oxygen that quickly. Similarly, regular exercise below the 60 percent mark has little sustainable impact — though for beginners, it's a good place to start.