I knew that "stopover" in the US is 4 hours, or the time to the first flight on the next day (24 hour limit notwithstanding).

I'm trying to work out if different definitions of "stopover" apply to Europe, or for tickets that originate in Europe, as opposed to other zones.

I think that the US definition was probably concocted to stop people buying a ticket with a crazy routing that allowed for a meeting every day in a different city, but only pay a single $400 fare. Which is, of course, kind of what I'm trying to do (albeit with a somewhat more expensive fare).