how often should coolant be changed?

Check your owners manual. If you have the Toyota Long Life coolant (red stuff) it should be changed at 100K miles and every 50K miles thereafter. If you have aftermarket, it's usually 50K miles or 5 years whichever comes first. Check the label.
If you want to keep the car as long as possible do it every three years. The coolant does gradually turn acidic and eats at things like head gaskets. I believe the recommended interval is really stretching it.
BTW there is a 10mm drain plug in back of the engine that releases trapped coolant. Just draining the radiator leaves about 1.5 quarts of fluid in the engine block.