I’ve long held that a story I’ve started writing must reach a certain “critical mass” in order for me to carry it through to completion. This critical mass might be defined as the written length — the word/page count — beyond which I will not, can not stop writing. A story which might become a novella (or longer), for instance, seems to hit critical mass at about 10,000 words. At this point, the narrative has so much momentum in my head that it’s a force in its own right: I know enough about the characters and the story to know how little I really know about them, and how much I want to find out.
(I’m pretty sure many/most other writers feel the same way, incidentally, although we’ve probably all got different critical masses.)
Mostly, this is just a descriptive figure, not a prescriptive one. It’s an observation which seems to have held true in the past and (as I expect) will likely hold true in the future, not a make-or-break target.
For the piece I’m working on now, though, I seem to be approaching — perhaps have already crossed — a different and perhaps unquantifiable threshold: a critical mass of research.