This might help you see the relationship between an epics and stories. Consider the literary definitions. Star Wars is an epic. Within the Star Wars epic there are many mechanism for telling the story (movies, books, TV shows, animated shows, etc). Each of these mechanisms can contain more than one story line and those story lines can span multiple occurances of mechanisms. Each of the mechanisms for telling the story require different amounts of work to produce. The total effort to produce the Epic is made up of the sum of all the story mechanisms. An Epic can live on for ever or could be completed. But how long it takes for the completion or even if it will ever be completed is not known and is determined after the work to tell the story begins.
So, given that description can you arrive at a reasonable answer to your own questions?