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

I let go for any of the following reasons:

a) I become bored and need to start something new (this doesn't happen often).

b) My game becomes too annoyingly bugged to continue with.

c) I feel I need a clean start to incorporate a new expansion (though, I will usually transfer favourite sims to a new game anyway).

I let go when the save file just can't go on anymore, like it's too bugged or corrupt to function properly. But specifically for stories and not save files, I just play a few across the save. So if I get bored of dealing with my main family, I can work on their neighbor's storyline. Sometimes, a break is a good way to get your muse back for the old story, or to give you inspiration on how to end it in a satisfying way.

Like once, my sim was dating an man and they lived in an apartment, and she snuck out to Mt. Komorebi to earn money a snowboarder. I didn't vibe with the story, so I played with my sim's sister a bit. I liked the sister a lot more and focused on her family, and the first couple found a solid spot in the story as supporting characters. 

So yeah. I think a good idea is to put a story to the side and try something else. Either absence makes the heart grow fonder, or you find something better.

For me it's not so much let go of a story but rather I get new ideas for a new story I would rather play instead and when that happens all other stories are left there waiting for my return. Sometime I play with the sims indirectly by befriending them with my other sims and what not. 

I only let go when a save file is so bugged/currupted that it can not function. I do however attempt to save the family (hoping they are not the cause of the curruption) and put them in a new save. That option unfortunately erases generations of sims, but it is sometimes need to give a clean slate.

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.

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

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.

I typically have multiple storylines within the same save, so, for me, abandoning a story and abandoning the save are two different things. I never abandon saves. If I'm tired of a story, or a challenge, I move on to a different one, but I sometimes return to previously abandoned stories.

I'm actually considering starting a new save because the one I'm currently playing is so buggy that it's keeping me from doing all the things I want. 

My biggest issue with bugs right now is missing and inaccessible lots. Also, I noticed a whole bunch of my townie families are... missing??