For German and US aircraft, its not heat-based, its just a matter of running out laughing gas or water mixture to inject into the cylinders.
The P-51D used water-injection IIRC, and so it too should have a set limit on WEP time. I think its the same for most if not all US fighters, though I'm not 100% sure.
The UK just ramped up the boost pressure on the superchargers, and I have no idea how the Japanese or Russians did it.