I recently had the experience of a story, Opening Earth, coming alive as I wrote it; in this case, it came alive as I was finalizing it.
I was finishing Chapter 8 and thought I would conclude the story in about two more chapters. Then I had a good idea, which led to a second idea, and so on. I am now working on Chapter 19 and think three more chapters will finalize this part of the story. The story will not be 'finalized' for I will leave a number of incomplete subplots and I have several more 'good ideas', but I decided to cut off the story now. Depending on how it is received by SOL readers, I may write the remainder of the story.
However, I have also posted two stories as Part 1 of multipart stories. Once I finish Opening Earth, I will return to those stories and write the Part 2 of each. There will be no Part 3 for Chocolateen. Ancient Abilities has the potential for several more parts, but I will have to see where I take Part 2 before I decide if I want to continue the storyline.