I usually get everything I intend to write done in less than an hour, whether that be a short 500 word blurb or even a 4k word chapter. I've done some of the latter in less than half an hour before work.
When I get inspired, it all comes easily. I usually pick up an idea for a story and spend about a month pondering the details, everything from character motivations to possible interactions, to larger things like overall plots and themes. Once I've got all of that together, it's relatively easy to actually just start throwing it out into print. I rarely even stop to edit, aside from simple corrections as I type (a bit of a flaw, but not one I'm keen to work on fixing a the moment.)
Generally, in the first few day I can get several chapters done in a single sitting. After that, things start to slow down as the high wears off. (I've begun to think of these creative bursts in the sense of an actual physical drug due to what they actually do to my mental state). Eventually, it becomes too difficult to continue writing, and my pace slows down. Even though in my mind, I know how everything I've ever written ends, I have issues actually putting fingers to keys on most of my older works.
So yeah, I've done crazy things like 13k words in a day, but those are usually at the beginning of a writing spree. On average? I rarely write for more than an hour at a time, but the time between writing attempts tends to grow as the story ages.
