Just as you would change the oil in your car every 3-5K miles, or paint your house once every ten years. The roof will need to be treated at least once every three years. We have been in the business for over ten years as honest contractors and you are dealing with myself or my RMI whom are both knowledgeable and easy to deal with. We see how long it takes for roofs to get dirty again after a cleaning as we have done thousands of roofs and on going maintenance.