As a reader, I'll read anything that's done and done well. I'm sure there's an upper limit somewhere, but I'm not sure what it is. I'm currently knee deep in an 89k-word fic. I just wish AO3 had a way to let me mark where I left off because I have to just kind of remember.

As a writer, I guess it's just whenever the story is done. So far, that's been in 14.4k words or less.