The short, unhelpful answer is: stop when it feels right.
That means relying on your story instincts. Don't rush things, don't drag things. When the story's complete, end it. If it's not yet been told, don't end it. But don't kid yourself into thinking it's not over yet either and writing onward just because you don't want it to end. And don't jump the gun and end it too soon either.
Goldilocks.
More concrete advice would just be to write the story, and worry about where to end it later. You can always trim off. But there has to be something to trim first.
If I have any beliefs about immortality, it is that certain dogs I have known will go to heaven, and very, very few persons. —James Thurber
^ Seconding Rach above me. You really have to look at the pace of your story, but, in the end, a story is finished when a story is finished and there is no right answer.
You know you're a writer when you're not alarmed at hearing voices in your head, you can't read a book without analyzing it for plot & characters and you consider something you nearly killed yourself to write the most rewarding.
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