Just make sure you can touch the lower radiator hose with your hand and hold it thgere for a min or so.
If you drain all the coolant out of a hot engine you run the risk of cracking the block.
Same thing if you add cold water to a hot engine, either by washing it or refilling the cooling system.
It does not need to be stone cold, just cool.
A hour of cooling off time is all that is needed, less if it is cold out side.

 

So I read 15-20 minute cool down before adding water and draining again. Now I am reading that I should let it cool down for an hour?

Can someone please clarify?

Thanks.

 

15-20 min is plenty, a hour is overkill, the water can still be warm just not hot.