It's a marketing category and it's as vague as the seller needs it to be. In its original meaning, as with the Epic of Gilgamesh, it's a telling of a generally known story (the "spoiler" idea is way off-beam, all the audience knows the plot in advance) and structured in a way that reading, recitation or singing are all ways to communicate it. So virtually no modern epics exist. (Multi-generational stories are hardly ever epics: the Iliad, Odyssey, Æneid and Epic of Gilgamesh all take place in no more than ten years).

I can only think of one story in recent decades that comes near to the classical model of an epic - Ngugi wa Thiongo's Devil on the Cross. It was the first novel ever written in Gikuyu, and since literacy in Gikuyu was so rare when it came out, storytellers took to reading it out in bars, and got paid for it. More like Homer than an audiobook.