We have to go by hints and guesses to an extent, because Brandon never went into spoiler territory and spelled out that Jordan wrote this and that, but from what Brandon has hinted at, Jordan has pretty much written in full the finale of the Last Battle at Shayol Ghul and the epilogue section, and that's about it except for fragments/chapters/scenes. The epilogue section (including the last scene which Jordan has put in writing shortly before he died) we're sure of, as Brandon has referred to that specifically a few times. He also has strongly hinted that Jordan had advanced a lot the final confrontation as well (we can guess it's the SG events he means). The climax of AMOL and the beginning of it (the prologue) is what he left the most completed or near completed writings for, the rest is scattered scenes all through the book (he notably wrote some Mat scenes which are in TOM), and various sorts of outline. At a signing I attended, Brandon said we might be surprised by the parts Jordan tackled first and advanced the most, that they were often not what the reader might think as "key scenes" for the plot but rather "character moments", including many dialogue-only outlines.

Brandon once said Jordan had left virtually the entire prologue (that got split between TGS and TOM and added to by Brandon, and some scenes he moved into the books themselves too, IRRC), and about the same length of near completed writing for the ending. So AMOL (as book 14, I mean) is not quite finished, no more than the other two were. Brandon most likely has as much work to do on it as he had for TGS, and that if the last book isn't substantially longer than TGS, like TOM. He onced said that the third book might end up being the easiest to write, because threads are all coming together (not only the characters but in story terms as well) and he has less in it to figure out than in the first two thirds. But that was said long ago, even before he started to realize the book was a lot more complex, and would be a lot longer, than his early impressions of the AMOL outline had made him think. Presumably, that's as true for AMOL as it was especially for TOM (the scope isn't likely to shrink back for the climax, if anything it will keep expanding a little, with the full main cast and the full secondary cast, unlike TGS)

Brandon's assistant commented a short while ago that while Brandon would have liked to stick to the 2011 date for the release, it will more realistically come out in the first or second quarter of 2012.