When we get down to it, it's as indefinable as the term "Classic".

Even so, I'd class Aguirre, Wrath of God (1972) an epic, even though it's at 93 minutes. In fact, it's the shortest epic I've seen, and one of the best films ever made.

Without researching into it, since the term is, as I said, an elusive one, I'd say an epic to me was a film with grand themes which go beyond somebody dying or making us laugh. Which is why I'd never call a Marx Brothers film or a Tarantino film an epic. Epic also indicates a collaboration piece: not, say, a character study of one man (though Ben-Hur is), but with a cast of thousands. It's got little to do with running time: normally, though, the length of a film happens to go in accordance with the themes, characters and narrative it portrays.

Thanks for reading.
Mick