Apparently he's written the beginning of Winds of Winter (from the Starks point of view) and at least titled A Dream of Spring (apparently the last of the series) so I'd hope that means not too long.
I don't know how much you can read into that, when you consider that a large chuck on Dance of Dragons is just what he cut from Feast of Crows; and it took him six years to release it!
Infact, it says this at the end of Feast for Crows (I've edited out the names to prevent spoilers for others).
"'Hey, wait a minute!' some of you may be saying about now. 'wait a minute, wait a minute! Where's **** and ****? Where's ****? We hardly saw ****. That can't be all of it...'
Well, no. There's more to come. Another book as big as this one.
I did not forget to write about the other characters. Far from it. I wrote lots about them. Pages and pages and pages. Chapters and more chapters. I was still writing when it dawned on me that the book had become too large to be published in a single volume ... And I wasn't even close to finished yet. To tell all of the story that I wanted to tell, I was going to have to cut the book in two.
The simplest way to do that would have been to take what I had, chop it in half around the middle, and end with "to be continued." The more I thought about that, however, the more I felt that the readers would be better served by a book that told all the story for half the characters, rather than half the story for all the characters. So that's the route I chose to take.
****, ****, ****, **** and ****, **** and all the rest of the characters you love or love to hate will be along next year (I devoutly hope)
in A Dance with Dragons
So what should have been one year later turned into six, even though he had already written a fair chunk of it. So god knows how long The Winds of Winter will take.