What is your definition of a "classic" film and what is the last classic film?

[MechaGodzilla]

A work of outstanding quality which endures the passage of time. Of course everything shows its age one way or another (which isn't a negative, contrary to what some believe), but certain works stand out in that they retain their appeal and the high standard of craftsmanship and artistry remains apparent while their contemporaries begin to look decidedly creaky or fade from public memory.

And, of course, part of the definition of a classic is also that it becomes a standard against which others are judged. Films like, say, Citizen Kane, Casablanca and the works of Hitchcock certainly fit the bill. Several generations of films to follow the aforementioned were and are still compared to these classics. How often don't you hear "it's no Citizen Kane"? Or how directors of thrillers are sometimes praised with "he's the next Hitchcock".

Now, what I've always had some trouble with, is determining how much time has to pass before something (in this case a film) can well and truly be declared a classic. Is it 20, 25, 30, 40 years? 50 or even longer?

And then the fact that film is, obviously, more reliant on technology than, for example, literature. Does that mean they date faster, and thus could earn "classic status" sooner? Is it more impressive for a film from the '70s to still hold up compared to, say, a novel of the same age?