(HULK) said:
Per wikipedia:
Martin believes the two last volumes of the series will be big books of 1500+ manuscript pages each.
[20] By October 2012, 400 pages of the sixth novel had been written, although Martin considers only the first 200 as "really finished", the rest still needing revisions.
I just read that as well. I cannot find the quote I thought I had seen earlier. GRRM has a tough task because he doesn't always write chronologically and he switches points of view. I read somewhere that before he begins writing on any given day, he re-reads and edits everything he wrote the day before. Also, when changing points of view, he goes back and re-reads all their POV chapters in the new book, plus many from the previous novels. Plus, he has to constantly check his facts against what he's previously written. So basically, he is reading more than he is writing, when he is writing.