Is there a scale that details the criteria that needs to be met before a movie can be classified as a "classic"? Are there mathematical equations that when tabulated and cross-referenced with a chart of quality to help us in naming a movie a "classic"?
The truth of the matter is, the designation of "classic" is somewhat subjective- the definition varies from reviewer to reviewer. With that in mind, I will do my best to give you my definition of what a "classic" is...
First, let's consider the influence the movie had on the industry and genre. Did the movie change how people looked at horror movies? Did the movie change the way director's, etc shot and edit their films? Did the movie change the way that technology and special effects were used in movies? Did the movie inspire others to get into the film industry? Did the movie change the way director's etc think about the various storytelling elements involved (ie- did it inspire more character driven stories, or inspire more "found footage movies")?
If the movie has that sort of impact on the industry and movie experience, then the movie- in my opinion, certainly deserves to be considered an influential movie.
Another thing that needs to be considered is the impact on the population. If the movie sparked debate and conversations over a wide span of time regarding the motivations of the characters involved, as well as the assumed themes of the story, I would say it had an impac ton the population. It would be safe to assume that the movie's impact on society was considerable if the movie is analysed as a reflection of society in college courses.
Both impact on the industry/genre and on society are important for an influential movie... but doesn't make it a true classic in my mind. For that to occur, there is one third element needed.
Does the movie make you want to watch it more than once over a several years? Sometimes, for me, this trumps the other two requirements. I have seen influential films that are hailed as "classics"... but never felt the desire to watch more than once. On the other hand, I've also seen plenty of not-so-influential films that I've watched several times in a year because I wanted to.
So there you have it- my definition of a "classic":
"Any movie that has had over a period of time considerable influence on the industry/genre, collective psyche of society and retains the ability to create the desire to rewatch it over that period of time."