When it’s something I’m excited about and I get started right away.
If I get what I think is a great idea and immediately begin working on it, odds are I’ll be done within a day.
Depending on the scope of the project, of course.
This is how I’ve handled a lot of short story ideas.
Idea manifests while lying in bed at 11pm, first draft done by 2am.
On the other hand, I’ve also been struck by a great idea at 11pm and thought to myself, “That’s amazing! But I’m too tired to work on it now. I’ll wait until morning when I’m refreshed.”
Come morning, assuming I even remember the idea, I’m scratching my head trying to figure out what I was so amped up about the previous night, and ultimately nothing comes of it except a vague sense of misgiving.
I also feel productive when my job’s at stake if I don’t get it done (funny, that) and when I’m in the shower and can’t possibly act on all the ideas I’m having (let alone write them down).
The rest of the time, I’m just trying to sleep…