This is one of those questions, like “Where do you get your ideas?”, that people will keep asking writers – and like the question about ideas, it’s one that doesn’t have an answer, or at least not the kind of answer the questioner is looking for.
Even before I was a novelist, people used to ask me how long it took me to write my dissertation. The only answer that I could give them – “Well, if you look at it one way, it took me three years. If you look at it another way, it took me about three very intense months. But I needed the three years first.” – somehow never really satisfied them, even though it was true.
The short story I finished just this past weekend is much the same. In terms of actual putting-words-on-screen writing time, it took me about a week. But this was a story for which I’d had the title and the central conceit rumbling around in my head for a lot longer than the three years it took me to write my dissertation back in the day – and not until last week did it take on enough shape that I could make a narrative of it.
(Why then, after all that time? I have no idea. But when the Muse shows up with a plot and a theme in hand, only a foolish writer turns down the gift.)