Do you need to change coolant in radiator every 2 years or can you check the antifreeze level in radiator with a hydrometer. If so what reading to you need?.
Most antifreeze contains rust inhibitors, that's why it is advised to keep it in the system all year round, its not a bad idea to change it periodically. Be careful using an antifreeze tester on a remote header tank sample as they sometimes do not always represent what is in the system, most hydrometers these days give a red, amber, green for the test or give a lowest protection figure.