This is a slightly difficult question to answer, as the amount of time between flushes depends heavily on what kind of car you have and what kind of coolant your car uses. You should be able to find the coolant replacement/flush interval in your owners manual or service manual.
In terms of the flushing procedure, this is what I like to do:
- Obviously, start by emptying all of the old coolant (generally there
is a drain plug at the bottom of the radiator).
- Then, fill the cooling system with distilled water, optionally you can
add radiator flush chemical as well.
- Start the engine and allow the water to circulate through the car, racing the engine a little until it gets up to temp.
- Empty the water and refill with your new coolant.
Steps 2 and 3 can be skipped if your car has a short coolant replacement interval, but I'd recommend to always do them if the car has a long coolant replacement interval or if you are changing to a different brand of coolant.