For "usual" antifreeze, the vehicle manufacturers usually advise coolant modifications every two to three years or 30,000 miles. But it is also not bad to change coolant every year for highest deterioration security especially in vehicles that have blocks, aluminum head or radiators. But such suggestions will soon get outdated. A number of antifreeze suppliers have recently set up "long life" antifreeze formulations that claim to offer security for four years or 50,000 miles.

General Motors has introduced new five year, 100,000 mile antifreeze in its 1996 cars and light trucks. The fresh coolant is called "Dex-Cool" and is colored orange to differentiate it from regular antifreeze.
CAUTION: These new long life coolants offer more life when used in a fresh system mixed with water. If combined with usual antifreeze or old coolant in a system, the decay protection is decreased to regular antifreeze.

Corrosion Inhibitors
The life of antifreeze depends on its capability to reduce corrosion. Phosphates, Silicates and/or borates are applied as corrosion inhibitors to keep solution alkaline. The time in which antifreeze is left so, deterioration held in check and you are not required to modify coolant. But as corrosion reducing chemicals are expended over time, electrolytic corrosion begins to corrode metal inside radiator and engine. Aluminum is particularly weak to decay and can rely on Swiss cheese rather than rapidly if conditions are good. Solder bloom can also form in brass/ copper radiators causing restrictions and leaks. So changing coolant occasionally as preventive protection is a batter way to avoid expensive repairs.
So it is better to change the coolant before corrosion inhibitors arrives at seriously low levels. To keep corrosion in check, following OEM change suggestions is much better but situation can vary in different situations. That’s why, several changes may be recommended to lessen the risk of corrosion in aluminum radiators and bimetal engines.
 

Checking The Antifreeze                                                                              Testing the anti freeze is the best way to check is it the time to change Antifreeze or not. Special antifreeze test strips are introduced by different suppliers to react to change color and pH of the coolant. If test strip specifies a bad or minor condition, it means you should change the coolant.

Changing The Coolant
To change the coolant, reverse flushing is the most suitable way because reverse flushing helps to remove deposits and balance that can deal with good heat transfer.
CAUTION: Avoid using more than 70% antifreeze or running straight water in cooling system because it presents no freezing, corrosion or boilover safety.