Twelve to seventeen minutes is plenty on the treadmill--if it's done fast. That's all you need for cardiovascular benefit. You don't need to spend that extra time unless you are over weight and you need to burn off extra calories. Do it vigorously, like somebody is chasing you. You've got to do it hard. Otherwise, if you just take it easy and do it longer, you are spending all that time when you don't need it. Use that extra time with your weights instead.