According to the ACSM, individuals should aim for a minimum of 20 minutes of moderate intensity exercise per day, for 150 minutes per week.
For those who are obese and try to lose weight, or anyone looking to keep weight off, ACSM recommends 200 – 300 min per week (3 – 3.5 hrs.) or 1 hour 4 – 5 days per week.
Walking Intensity:
Not all walks are created equal. It is important to make sure your heart rate reaches a moderate intensity level during your walk. Moderate intensity exercise is defined as an activity that raises the heart rate 50 – 70% of maximum heart rate.
If you decide to up the intensity either by adding resistance training in the form of weights or including short periods of running – exercising to an vigorous activity level (75 – 85% of maximum heart rate) requires the during of your walk to be cut in half to achieve the same benefits.
In other words, a 60 minute moderate intensity walk is the same as a 30 minute walk/run of a vigorous intensity level.