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

I am still playing the same sims since the game launched. The problem is that we have new game play packs every 3-6 months. So when I new pack comes, I create a new save, or make a copy of an existing to tryout the new pack. I get back to my new sims to only play them for about a month and I am soon looking at a new pack. I think their story will end after EA stops making expansion packs.