It's not a solid number; it can be anywhere from the late 5's to the early 6's. Apparently, also, mod's can push the redline up a little bit; I think I remember hearing Venom's omgDuratec hitting 6500 or so, and jtzk13's hitting 7k.
My (stockbutold) Duratec regularily hit's 6000; that's also the printed redline on my Tach before the redline was removed. I would put a max at 7 8k and a redline at 6.