The showrunners have already stated they want the show wrapped up in 7 seasons.KingsGambit said:As such, I'm approximating that it would most likely look something like the following:
Season 1 - Book 1 - 2011
Season 2 - Book 2 - 2012
Season 3 - Book 3a - 2013
Season 4 - Book 3b - 2014
Season 5 - Books 4a, 4b, 5 - 2015 (all three books mushed together)
Season 6 - Books 4a, 4b, 5 - 2016
Season 7 - Books 4a, 4b, 5 - 2017
Season 8 - Books 6a & 6b - 2018 (assuming they occur in the same time-frame and not sequentially)
Season 9 - Books 6a & 6b - 2019
Season 10 - Book 7 - 2020
[Season 11 - Book 7b - 2021]?
http://screenrant.com/game-of-thrones-final-season-7-8/
Even I think that's unlikely, and it's possible we get a "7th season" that is actually 20 episodes long and split over two years, the way Mad Men has done, and the way Walking Dead loves to split "a season" with a 4-5 month hiatus in the middle of it. Best case scenario is we get that 8th season, either officially or through a bloated 7th.
Book 4-5 material will take them through season 5, if they remain largely faithful. Given their desire/need for set-piece events, I rather suspect we get an event from early Winds of Winter in season 5.
Either the battle for Mereen or the battle for Winterfell. The former has the potential for being a glorious clusterfuck, even with the almost complete removal of the Ironborn from the show's narrative.