Same thing on the book itself. The book is done when you've finished telling the story. I've found that the more I write, the more likely I am to have a complex story with a well-defined beginning, middle, and end pop into my head all at once.
After that, I plot the whole thing out and start trying to fill in details to connect the pieces together. When I work on any given part of the outline, I keep reminding myself to write until its finished and don't rush it. What happens is that I get into a groove of how long my chapters are (and I tend to write shorter chapters) that I shoot for going forward.