If someone sends a completely new email (compose instead of reply) that happens to have the exact same subject as another thread, it's still considered a new thread. The system figures this out by mainly by reference information encoded in the email headers by everyone's email programs. There is no time component.
Hmm, using the absence of the In-Reply-To or References fields to mark a new thread has the problem that neither field is mandatory. I don't know if there are any major email services/clients that don't support one or the other (or both), but that was the usual motivation for using "Or" logic rather than "And" (the subject matches or the field matches an existing member of the thread).
But maybe that's no longer (much of) an issue. I like the way using "And" neatly solves the "stale thread" problem without having an ad-hoc time limit.
