On every coolant container, there are guide marks that indicate how much coolant is left in it at any given time. Keep an eye on these guide marks and refill the coolant as soon as the level drops below these markers.  Experts and automobile manufacturers recommend that the coolant container be refilled every 30,000 miles. However, keep in mind that your car may require a refill much earlier than 30,000 miles or much later than this.  The condition of your car, the components of your car, and your driving style all have an impact on the coolant backup in your car. Be cautious about it.