The average writer can pump out at least 4 pages a day or 1,000 words. Some professional authors can write 20 pages a day. Stephen King used to blaze through books. This is how your favorite author can release a book every year. But, let's keep it at 4 pages. 4 pages over a year is 1,460 pages of material. That's a little bit larger than the size of a typical ASOIAF book. Then, let's give him a second year to completely rewrite it and fix all the flaws in the story. Then, let's give him a third year to have it professionally edited for commercial release. Also, let's give him a buffer year to relax/travel/plan out the story. Going by this, we should've had Winds of Winter by 2014 or 2015 and A Dream of Spring by 2017 or 2018.
Dude has taken longer and longer and longer to release each book.
1996 - Game of Thrones
1998 - A Clash of Kings
2000 - A Storm of Swords
2005 - A Feast for Crows
2011 - A Dance With Dragons
2017, it's almost 2018 now - ?????
He went from dropping these joints every two years to taking 6-7 years.
So either, the first couple of books must have been written BEFOREHAND in the same way Tolkien had wrote the entire LOTR trilogy or GRRM has lost motivation to finish the story overtime because his output nosedived. I wouldn't be surprised if the new book is still unfinished
I really think he wrote himself into a corner and he realizes he can't finish the story in 2 books. With all the storylines he has (Aegon, Arianne, Dany, Jon Snow, Tyrion, Margery, Bran, Sweetrobin & Sansa, Theon, Yara & the Ironborn) up in the air, he realizes he needs to write 3-4 more books instead.