How long does it take for a radiator to cool down?
Hereof, how long does it take for a car to cool down after overheating?
If your car's overheating, pull over, switch off the engine and wait for at least 30 minutes for the engine to cool down. To be on the safe side, it's preferable to wait an hour or two to fully let your engine cool.
In respect to this, how long should you wait to open radiator cap?
Typically though it should be left to cool for 1-2 hours before it's cooled sufficiently to open the rad cap safely. If you're actually trying to check the coolant level then you probably need to wait more like 3-5 hours (as if the coolant is still warm it will show a falsely high reading due to expansion).
In instances where an engine has overheated, causing a breakdown, there may be a need to add fresh coolant/antifreeze to the cooling system. However, you should never add coolant/antifreeze when the engine is hot, and instead wait for it to cool.