How often should the radiater coolant be changed

Every two years. The antifreeze is an alcohol and will always be an alcohol. It's the additives in it that wear out, and acids form in it from the normal seepage of combustion gases into it. Replacing the coolant gets the acids out and new water pump lubricant and corrosion inhibitors in.

GM used to advertise their coolant was "lifetime" to make the cost of maintenance appear lower than for other car brands, but right on the reservoir it says to replace it every three years. Even that is too long. They use "Dex-Cool", (we call it Dex-Mud) because of all the problems it causes. Even the Dex-Cool manufacturer doesn't recommend waiting three years.

Other manufacturers use different antifreeze formulas that are denoted by their different colors. Be sure to get what is recommended for your car.