I move on when the story is done.
For some saves, that means once my initial sim has accomplished some goal (usually an aspiration for Test Saves)
For some saves, that means after so many generations have done their thing... whatever "their thing" may be. This style of informal Legacy play is my most common.
And yes, for some saves, that means when I say "Nope. Not interested in this save any more."
I've also been known to jump between saves, usually my current legacy save and whatever short term save (testing, scenarios, etc) I have in mind. This usually keeps saves from getting too stale in play.
