When do you start playing a new sim?

Only for the scenarios. Otherwise it's my same family I've been playing for years.

When I finish telling their story.

I play my own homebrew version of legacy challenge, following a family through the generations, so I play Sims until they die...but not at my hands. Usually of old age, but also sometimes due to ... other events.

But I also switch to playing their children as separate households once the children have grown because I don't leave them in the same legacy family house, but rather send them out in the world, so at that point I'm also playing rotational, at least until the older generation dies. Or even after they die if the legacy heir has siblings which I rotate between.

I start playing new sim(s) when I create a new story. Sometimes, new packs inspire me, but not every time.

I rotate between my different households but without a certain milestone, usually just chose a household to play when I start the game.

I play rotationally. Sometimes I play a household for days, sometimes only for 10 minutes, so it's always different and depends what part of the game I want to play. I don't think I've ever gone more than a week without switching lots though, but I used to play generations of the Goths, Calientes, Burbs and Brokes in Sims 2.

When I've completed whatever story I wanted to tell with them.

What that story is, I figure out as I go. If a given generation of a particular legacy ends up without a compelling story, I'll move on to their kids (or one of their siblings, or one of their sibling's kids.)

My option - When they have kids - should have been - When they have kids I move them out and switch to them - .

A bit of "All of the above" at times, but mostly it depends on the sim I'm playing at any given time and how invested I am in their ongoing story. My "play" is really a creative process for stories, so sometimes it's a little like work. If I get bored, it's usually because either something in the story isn't working or non-story (and non-game) factors are invading my playtime. If it's story stuff, I do something to shake it up and see what falls out. Sometimes that means making new sims to throw into the mix, and sometimes it just means solving puzzles of a sort as I search for ways to make that particular story better.

I would like to make stories too, but it's not that easy with the options we're given. Or I lack the imagination. That's why I enjoyed Strangerville so much. I think they should release more content like that, even if they're really small chain quests like a proper mmo would do. So far stories in my game are all about becoming a respectful member of society. It gets boring. I remember I had the opportunity to steal a contestant's pie at the fair, that would have steered the story somewhere interesting, especially if I had been caught. But I don't want anything bad to happen to my sims! They don't die, they don't get demoted, they help their roommates with their needs, they stay young, they become successful, the end. Then I create a new sim.