When the scene or scenes reach a ‘natural’ jumping off point, ideally ending with a tantalising question or clue for the next chapter - or if alternating between POV’s, the next chapter that is in that character’s POV. This way you give the reader a reason to keep reading.
Using Intrepid_Fortune_1’s example, personally I would do this: “Things are going exactly as I planned!” But things did not go as planned. Not at all.
End of chapter -
This way, you leave your readers with a reason to read on by posing a tantalising question. Why didn’t things go as (POV character,) planned? What happened? And perhaps even more importantly, how does it affect the character?
Some writers write first and THEN organise things into chapters, and that’s valid. Personally I prefer these days to have a rough chapter outline or synopsis, so I know where I need to get to by the end of it, and how it will link in to the main narrative. Story structure and plot points can help with this, but a good idea of both where you’ve come from and where you need to get to generally gives you an idea of where to stop.
Not every chapter needs to end with a cliffhanger - which is the tantalising question taken to the extreme, but ideally there should at least be the question, “And then what happened?” in your reader’s mind when they reach the end of a chapter. That’s part of what makes a book a page turner, rather than something your readers feel they CAN put down.
