What does the manufacture recommend? For example mine recommends replacing the coolant every two years. Unless I am having a problem, I drain the system as completely as possible and flush the system with tap water (including a flush directly through the heater core, being careful to use low pressure). Once I have all the old coolant and crud out, I flush several gallons of distilled water through the system to get rid of the tap water then refill with a 50/50 mix of OEM coolant. If you are having a problem cooling, you can also try a citric acid flush, but it might just be time to replace the radiator.