I've occasionally encountered some difficulties, when discussing films and saying 'It's a classic'. Some people don't consider films from 90s as classics, they have their own proof like 'It's not as old as it should be to become a classic'. So does the film need to be old enough to be rated as classic or...? And what are your criterias of classic film?
If society still likes something long after they would have generally forgotten about it, it's a classic.
That was always going to be hard to define. Personally, if you're still raving about it ten years later, and you know that a good portion of the populace agree with you, it's a classic.
Let me hazard an opinion: a "classic" is something that has outlived its contemporaries.