It seems obvious: you stop a scene when it stops being interesting and pick it up again when a new scene starts being interesting. 'cept that there are complications.
For one thing, a scene break is a dramatic device.
Like cutting to a new paragraph (especially a short one).
But more so.
Unlike the paragraph cut, it lends more importance to the act, description, dialog that happened just before it. You want to end on a high note so that the scene break and the events described work together and not conflicting. Ordinary story time, with enough interest to keep someone reading to the next sentence, is not the way to end the scene.
The other thing in which it is like paragraph breaks or the length of sentences is that it's a structure device. Too many short scenes, like too many paragraph, or too many short (as in under six words) sentences make the story choppy. Fine if you want that, but its aesthetic usefulness is rather limited. It's true that you want a scene to get through what it's about as quickly as it can, for economy reason, but it's also true that you want to pack as many useful things into the scene as you can, which lets you limit the number of scenes and helps each scene not only move the story forward but do so in a manner sufficient to pull its own weight.
And if you have a lot of long scenes, a short one will stick out like a sore thumb. There's something disconcerting about discovering you've gone twenty pages without a transition and now you need to go to three or four page ones.