High temperatures and humidity are reputed to speed up the process. However, my stuff was in a fully climate controlled environment it's entire existence, so I doubt that was a significant issue.
I've experienced trucks coming apart while running, but it's hard to know when the damage was actually done. I know of two JLC GG1's that had their sideframes crumble while on a layout parked on sidings.
Zinc rot is baked into the mix at the time of molding, the only question after that is how fast will it happen.
