What's the latest in the year you mow your lawn?
Depends on how healthy it is and how early the consistently cold weather comes. I mow until it stops growing, then mow one more time a notch lower (this year, that'll be at 2.25" or 2.75"). Last year I had to mow mine halfway into December because I fed the snot out of it with slow-release organic fertilizers. I've been doing that again this Fall.
I'm hoping to get growth into December again this year, as I've got a lot of new grass again. The drought in June just hammered the lawn and I probably lost half. This fall was an overseed/spot seed with fescue and bluegrass (a little experiment that will probably fail, but it looks nice for now).
EDIT: FYI, tall fescue doesn't totally stop growing until soil temps get down near 40F four inches deep. In East Tennessee right now, it's close to 65F. So unless the weather gets really cold really fast, most of us can count on some more mowing.