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

I typically play a family until I get bored in some way. Most of my sims are eternal (I don't play legacies and I leave aging off) and eventually they max out every skill and have every achievement so I hit a "there's nothing for this guy to do anymore" wall and shift my focus. My oldest save has a lot of families that have become my forever townies instead of the randoms that the game makes.