I started to think about this while I'm revising a chapter in my novel. I think I wrote about 12-15k in a single session for some old fan-fiction project I did about 8 years ago. Damn near killed me too but I was so engrossed with the subject/story that I couldn't stop. Oddly enough, I can never seem to match that pace since. The most I've ever written in the last few months is around 3-4k.