To get back to CFH's point about the books being fragmented, Martin has stated that there will be no new POV characters aside from prologues and epilogues. I believe that book 6 will be mostly fusing storylines and bringing characters together, which in theory could finish the story by book 7.
Book 6 The Winds of Winter is predicted to be 2-3 years away still.
If he does finish it then it will be one of the strangest experiences for him. I imagine that JK Rowling just bowls about her mansion being bored these days, wishing she had something to write about that wasn't Harry Potter.
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