I must have missed where GRRM said he was writing ADoS concurrently with TWoW. I just googled it to make sure I really hadn't missed it, but all I found was speculation that he MIGHT be doing so, and that's what's taking so long. If anyone has confirmation, I welcome it.
He had about 200 to possibly 300 pages of Winter going by the time Dragons was done. Some of that was because parts written for Dragons were then moved to go later, into Winter, the bulk of that being done during the summer of 2010, when he was in the last stage of Dragons. He's been reading or posting some excerpts from Winter for the past several years, the last one a few days ago. Martin has said he's not adding any new viewpoints to the series. A chapter on Theon was released in December 2011 and published as an excerpt in the U.K. paperback version of Dragons, which I think was the earliest bit released, but I'm not sure:
http://grrm.livejournal.com/169899.html
This of course makes the calculations more difficult, if you want to make calculations, since Martin writes out storylines along each pov character, coordinating with what he has already done for other characters and what he has outlined to have happened. He originally presented the series as a trilogy, but as he writes the different lines and interacts them and juggles timelines, they've all gone longer than planned and the series went to seven planned books, the last two less likely to go long, since they are the most outlined out, I believe. So he's been in the situation where he's written plotlines and then cuts them off for a book, leaving the rest of the material for that plotline to be moved to the next book. So he writes concurrently and has pretty early on in doing the series. That's one of the reasons that Game of Thrones, Clash of Kings and Storm of Swords did go the way that they did -- because he was writing all three overlapping in time. He started Games in 1991 -- it wasn't published until 1996. That's several years. Then he cut a third of Games off and that third went into Clash of Kings, with some bits for Storm of Swords. Then Clash of Kings was cut and all that material got moved into Storm of Swords. So it actually took much longer to write the books than the publication dates suggest, because he wasn't doing them one after the other but in tandem a good bit of the time.
After that, Martin started writing what would have been Winds of Winter under the original plan. And he wrote quite a bit of it before realizing there needed to be a book in between, that the plotlines did not work and everything had to be scrapped. So that cost him several years. He then wrote what was both Crows and Dragons at the same time -- several years after Storm had been published. But it was going long and his publishers wanted him to split it. So he took the character plotlines that were done, split them off laterally and that was Crows. Everything else went into Dragons, which then required the coordinating of lots of plotlines around Danerys to finish. While he wrote Dragons, he wrote several parts along various character plotlines that went into Winds of Winter.
When Dragons came out in summer of 2011, Martin was contractually obligated by his publishers to tour internationally to promote the books and he was contractually obligated by HBO to tour internationally for the television show. Because the show had turned his series into a phenom with Hollywood adaptation -- for which they had him write an episode each year until he stopped that -- it was quite different than when Feast of Crows came out as just a bestseller. This is what happens the bigger an author gets -- they have a lot more obligations for promotion and events that take time away from writing. If George was the sort of writer who could dash off text at 4 am in a hotel suite, that would help, but not only is his writing process not conducive to it but the books have such massive casts and plotlines that it's very difficult to coordinate that effectively on the road, even in the digital age. And if he makes a mistake about what he's done before (emotionally, not just detail,) then he has to start again. So he didn't write the rest of 2011, did the promotion, and started Winter back up again in early 2012. He was hoping to finish by end of 2015 or early this year because of the tremendous pressure from the t.v. series. Did not happen, but does not indicate that he's oodles and oodles away from finishing either.
So if you want to calculate based on the past books, it's quite difficult because when he "started" each book is not clear, except for Games, because of the overlap and changes. Also, the books are not alike. Each one is different with different events and circumstances, some of which are easier to write than others. So a series average of 4 years, based on when Martin started writing the series (1991) to finished/published Dragons (2011,) would give you until 2019 for two books. Of course, with that, he missed the 2015 window, but if he gets Winter done this year, then, given that he's also probably writing bits of Spring as well, he might make it. But he might not.
It doesn't matter much since the ending of the series has been pretty known since the beginning and the television series will give you the basic details of it next year, even if they get there by a slightly different route, if that's what you want. But if you are going to calculate a book series end based on past history, then, logically, that the writing of the books overlaps does need to figure into calculations.
