I have heard it is best to change the antifreeze every 15,000 miles to protect your aluminum rad and heads.
"I recommend changing the coolant every 15,000 miles or so, even for extended drain interval coolant. As coolant ages it becomes acidic and can even hold an electrical charge. This charge can change the voltage of the engine temperature sensor giving a false reading to the engine computer. Old acidic coolant destroys the aluminum in the radiator and engine head. "
efficientmileage.com