Radiators typically last the lifetime of the vehicle and are usually never needed to be replaced unless it gets damaged or clogged etc.
But usually radiators last 100,000 to 500,000 miles and it's usually the transmission cooler inside the radiator that fails due to corrosion or rust and then leaks.
If my vehicle had 500,000 miles on it and the original radiator was in the vehicle still then I would say it's time to replace it just to prevent any problems.
If the transmission cooler develops a leak inside the radiator then transmission fluid will end up in your coolant and then potentially leak out of your coolant overflow tank.
That will lead to the vehicles engine overheating.
Most times people will replace their entire car or vehicle before they replace the radiator.