I finished a short story this evening, just barely making the deadline.
Looked at one way, I started writing it three days ago.
Looked at another way, I’ve been working on it for six months.
It took me most of that time to look at and discard all the possible stories I wasn’t going to write, and to find the right idea and the right angle of approach for the one that I was. And it took me the better part of a month to find the voice that I wanted to tell it in.
Once I had all of that sorted out, putting the words together was the quick and easy part.
So — how long is a piece of string?
And the answer is: As long as it needs to be.