How do you know when it's time to let go of a story?

When the story is done

When a game update/new pack would significantly change the nature of the existing story, prompting me to consider starting over

When I get bit by an idea for a new story while in a "grindy" part of my current one

I'm a storyteller by nature, even if no one will see the story but me, so I jump from story to story a lot, but I try to finish the old one first