For "ordinary" antifreeze, the vehicle manufacturers generally recommend coolant changes every two to three years or 30,000 miles. Others say it's not a bad idea to change the coolant every year for maximum corrosion protection -- especially in vehicles that have aluminum heads, blocks or radiators.  But such recommendations may soon be obsolete. Several antifreeze suppliers have just recently introduced "long life" antifreeze formulations that claim to provide protection for four years or 50,000 miles.  

CAUTION: These new long life coolants provide extended life only when used in a clean system mixed with water. If mixed with ordinary antifreeze and/or old coolant in a system, the corrosion protection is reduced to that of normal antifreeze (2 to 3 years and 30,000 miles). 

Corrosion Inhibitors

The life of the antifreeze depends on it's ability to inhibit corrosion. Silicates, phosphates and/or borates are used as corrosion inhibitors to keep the solution alkaline. As long as the antifreeze remains so, corrosion is held in check and there's no need to change the coolant. But as the corrosion inhibiting chemicals are used up over time, electrolytic corrosion starts to eat away at the metal inside the engine and radiator. Aluminum is especially vulnerable to corrosion and can turn to Swiss cheese rather quickly when conditions are right. Solder bloom can also form in copper\brass radiators causing leaks and restrictions. So changing the coolant periodically as preventative maintenance is a good way to prevent costly repairs. 
The basic idea is to change the coolant before the corrosion inhibitors reach dangerously low levels.  Following the OEM change recommendations is usually good enough to keep corrosion in check, but it may not always be the case. That's why more frequent changes may be recommended to minimize the risk of corrosion in bimetal engines and aluminum radiators. 

Checking the Antifreeze/Coolant

One way to find out if it's time to change the antifreeze is to test it. Several suppliers make special antifreeze test strips that react to the pH (alkalinity) of the coolant and change color. If the test strip indicates a marginal or bad condition, the coolant should be changed. 

Changing the Coolant

The concentration of antifreeze in the coolant also needs to be checked prior to the onset of cold weather. A 50/50 mixture of antifreeze and water is recommended and will protect against freezing down to -34 degrees F and boilover protection to 263 degrees F. 

For maximum protection, up to a 70% mixture of antifreeze can be used for freezing protection to -84 degrees F.    Then Again, Who wants to live anywhere it gets down to below -80 Deg???

CAUTION: Do not use more than 70% antifreeze, and never run straight water in the cooling system because it offers no corrosion, freezing or boilover protection.