You should change your coolant about every four years, Ibbotson advises. Coolant chemicals last longer than they used to, and newer engines aren't going to be damaged by leaving it in long.
When you do change your coolant, Ibbotson advises using a premixed formulation rather than adding tap water, which contains minerals that can cause trouble.
If your coolant says it should be mixed, use distilled water and don't use less water than recommended, says Sinclair. Coolants are designed to work best with a certain amount of water, and not using enough will make them less effective, not more.
If you live in an extremely hot or cold climate, your should give your coolant more frequent attention, said Sinclair. Still, it's something you should be doing on a regular schedule, at most once a year, not just when the weather changes.