I think the amount and frequency of exercise a person needs is partially determined by their lifestyle, age, and other factors. I walk back and forth to work 5 days a week, 15-20 minutes each way. I also have a job where I spend about half the day on my feet walking around and half the day at a computer writing. Because I have a relatively active job and lifestyle, I might not need as much exercise to maintain a healthy body as someone who is sitting at a desk most of the day. However, I recently lost 40lbs, so I have to be really careful not to gain that weight back... I am going to try working out three days a week, including running and strength training to maintain my weight loss. I put 5 lbs back on during the holidays, so my immediate goal is to get back to where I was before that!