I could stop to get the word count... But I prefer to count pages or content as a judge of how productive I am. I'm an abstract random sort of writer, because as soon as I reach a pause in the action, I'll stop to do a quick edit, then check my outline, and make sure I haven't missed anything before moving on, and that gets time consuming. But probably my most productive day was something like fifteen pages, near the very climax of action, and it came out beautifully...
I tend to get lost in the world, and what's happening, and especially if I'm writing for my favorite villian... Everything just picks up steam, and then finally I finish putting all the beauty and drama onto paper... And I'm shocked when I realize I'm on page 7, or 12, or 22.
*pause*
Okay, I just dug through my records, and my best day was 7.6 thousand words, but I usually average 3 to 5 thousand to get a good scene done. If I'm on a good writing day, I can usually get 4K words in two hours.
But I'm also writing with a handicap of having a life that leaves very little time for writing. It took me 6 months to write 225 pages. But I -will- finish... just slow and steady.