Coolant change interval is 10 years.
Why? (apart from the obvious reason of good-sounding work for garages to do). The coolant in our 2000 Focus is still clear and pink and cools the engine (and heats us) nicely. Any internal corrosion will have used all its oxygen up by now and so will have ceased. If you replace it, you will immediately introduce some more corrosive oxygen.(yes, I know it contains 'inhibitors, but...)
As for 'topping up', you shouldn't have to. About 2 yrs ago I put in 250mls of rain butt water (no corrosive chlorine) to replace the microscopic evaporation via the filler cap over the previous 14yrs. If starting to need regular 'top-up' I'd sell the car pdq.
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