When it comes to car seat safety the general rule of thumb is that they should be replaced every six years. However there are a few factors that can contribute to how long you can use a doona car seat. For example if the car seat has been in a car accident it is recommended that you replace it. Additionally if the car seat has been exposed to sunlight or other elements for extended periods of time it is also recommended that you replace it. Ultimately it is important to consult with the car seat manufacturer to get their specific recommendations on how often to replace the car seat.