It depends on the car, and how well it is cared for. Personally, I replace my coolant every 2-3 years. I alternate between flushing the system and just draining-and-refilling.
When I flush-and-fill, I flush the system with clean water. Chemical flushes are generally a last ditch effort to make due with a neglected cooling system. If you are considering a chemical flush, you might be better off simply fixing whatever is wrong, if anything.