How much aerobic exercise do you need to do to gain the benefits?

There are two physical activity guidelines in the Unites States. The first, the Surgeon General's Report on Physical Activity and Health, is a lifestyle recommendation. That is, you can modify it to fit into your daily routine and activities of daily living. The recommendation is that all adults should accumulate 30 minutes of moderate-intensity activity on most, if not all days of the week. The key words are "accumulate" and "moderate-intensity." Accumulate means that you can do 10-15 minutes at a time and repeat that a couple of times throughout the day; for example, 10 minutes in the morning, 10 minutes at lunch, and 10 minutes around dinner. Moderate intensity is equivalent to feeling "warm and slightly out of breath" when you do it. Recently there has been some controversy about the effectiveness of this guideline and its benefits. At the moment the recommendation stands, but we may hear more about it in the not-too-distant future.

The second recommendation is from the American College of Sports Medicine. The ACSM recommends 20-60 minutes of continuous aerobic activity (biking, walking, jogging, dancing, swimming, etc.) three to five times a week, at 60%-90% of maximum heart rate, and two to three days of resistance training. This is a more formal, "workout" recommendation, although you can also accumulate the more intense workout in bouts of 10-15 minutes throughout the day if you like. Follow this recommendation and your fitness and your health will improve.

Which one you choose is a personal choice. They are not intended to compete with each other but rather to provide options and maybe even complement each other. For instance, the Surgeon General's recommendation may be more practical for individuals who are unwilling, or unable, to adopt the more formal ACSM recommendation. Of course, there's no downside to working out regularly with aerobic exercise and also becoming more physically active as per the Surgeon General (take more stairs, mow the lawn by hand, park far away from the store and walk), so combining them might be a good decision.