If you do a drain and refill every 3-5 years, you will never need to flush the system. Flushing actually increases the potential for contaminants in the system. Every cooling system also has a sump in it that collects sediments from normal deterioration of the system, and flushing does not clean this out. Flush chemicals are very harsh on the seals and rubber parts of the system. Clear water flushes are less harmful but really don’t do any good, except if you are changing the type of antifreeze from an OAT type (DEXCOL) to the old “green stuff” or back. Its not needed with the latest HOAT types, which is most of the ones on the market today.
The old green stuff should be changed every 3 years for most engines, 4 years for all cast iron engines. The new antifreezes are good for 5 years or more. This is based on a complete drain, including the block. If you don’t drain the block, then you should drain and refill about twice as often. You should change all the hoses every 8-10 years or every other coolant change. A new thermostat with the hoses and a new radiator cap with the hose change is also a good idea.
The coolant will last longer if you mix it a little rich, two parts antifreeze to one part distilled water.