Something vast across a number of criteria. Time covered alone is not enough. A slew of characters alone is not enough, although they need to be bigger than life. Continents crossed on their own are not enough. And at the end you have to sit back and go "Wow" because there is so much to think about.

In all honesty I find it hard to consider stories an epic, unless they are really old and still beloved. I define epics in the narrowest way possible. Gilgamesh is an epic. So is the Illiad. Orlando Furioso qualifies as well in my opinion.

Moby Dick was just a novel for me. Not even one I liked. I just miiiight be willing to add Tolkien's Legendarium if we add modern books. I never read "Gone with the Wind" and am not very tempted to ever start, so no opinion there.

I suppose, ASoIaF or the Wheel of Time can qualify due to their scope. Never liked them either, but both series have this...vastness.