It's my understanding that it usually happens very suddenly. The neck just cracks and separates without any warning at all. On a car with that many miles, it's probably a good idea to just replace it as a matter of preventive maintenance.

I bought my 86 300E a year ago last June with 94,000 miles on it, and the neck had already broken and been repaired with JB Weld and a piece of copper pipe. I replaced it with a new Behr radiator with the metal reinforcing sleeve in both hose necks and replaced the standard MB anitfreeze with Evans NPG+ coolant and I've had no sleepless nights for the last 30,000 miles worrying about it.

Gary