To determine your maximum heart rate, subtract your age from 220. This number represents how many times your heart should beat per minute at its maximum rate. Multiply that number by 0.5 and 0.85 to determine your target heart rate range. If exercising at a moderate intensity, build up to exercising at 50% to 70% of your maximum heart rate for 2 hours and 30 minutes a week. If exercising at a vigorous intensity, build up to exercising at 70% to 85% of your maximum heart rate for 1 hour and 15 minutes a week.