I seem to remember it being written somewhere that it was six months...I even thought it was stated in the movie, but now I'm not so sure. Joker does say, "Let's wind the clocks back a year. These cops and lawyers wouldn't dare cross any of you," implying that a year before, Batman was non-existent or not yet considered a serious threat. Still, not that much time.

Anyway, now I think I'll have to make a list for TDK. I'll put it in my other thread when I have time, but now the official new thread police are complaining about too much Batman in that very thread, so maybe I'll hold off a day or too.