For most healthy adults, the Department of Health and Human Services recommends these exercise guidelines:
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Aerobic activity. Get at least 150 minutes a week of moderate aerobic activity or 75 minutes a week of vigorous aerobic activity. You also can do a combination of moderate and vigorous activity. The guidelines suggest that you spread out this exercise during the course of a week.Strength training. Do strength training exercises at least twice a week. No specific amount of time for each strength training session is included in the guidelines.
Moderate aerobic exercise includes such activities as brisk walking, swimming and mowing the lawn. Vigorous aerobic exercise includes such activities as running and aerobic dancing. Strength training can include use of weight machines or activities such as rock climbing or heavy gardening.
As a general goal, aim for at least 30 minutes of physical activity every day. If you want to lose weight or meet specific fitness goals, you may need to exercise more. Want to aim even higher? You can achieve more health benefits, including increased weight loss, if you ramp up your exercise to 300 minutes a week.
Short on long chunks of time? Even brief bouts of activity offer benefits. For instance, if you can't fit in one 30-minute walk, try three 10-minute walks instead. What's most important is making regular physical activity part of your lifestyle.
Source: http://www.mayoclinic.org/healthy-lifestyle/fitness/expert-answers/exercise/faq-20057916