The British publisher has already stated that there is 0 chance that book 6 (winds of winter) will be out in 2014. Lets assume for the sake of argument that book 6 will come out sometime in 2015. And 3 years later that book 7 (a dream of spring, it was originally called a time for wolves but it was renamed) comes out in 2018. (the first book, a game of thrones came out 18 years ago, so for 5 books that is over 3 years per book)

If they continue doing 1 season per year this is time timeline.

2014 season 4
2015 season 5
2016 season 6
2017 season 7
2018 season 8

HBO would want the show concluded by season 7 for season 7 is the last season before actors can renegotiate their contracts. If they have season 8, season 8 would be much more expensive than season 7.

The producers of the show say there current plan is 80 episodes for 8 seasons, they do not want to go longer than that for that means actors have contributed 8 years of their lives and they will be emotionally exhausted.

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George RR Martin supposedly has cleared his schedule and hunkered down to finish book 6, no tours, no side projects such as Dunk and Egg. He is also no longer talking updates at previous cons and his websites.

That said he is not close to finishing book 6 based on people that following him thoroughly and his British publisher saying it won't happen this year. (That means book 6 needs at least 6+ months to finish, since we are 3 months into the year, and it takes 3+ months to edit a book).