At the moment, however, it appears that Game of Thrones will end in June 2017 after seven seasons and 70 episodes, which puts book author George R.R. Martin and fans of the books in an awkward situation.
As of now, five of seven planned novels in the Song of Ice and Fire series (which Game of Thrones is adapting) have been published. The sixth book, The Winds of Winter, is underway with George’s publishers hoping to bring it out before the end of 2015. However, that still leaves the final book, A Dream of Spring, some way off. Bringing it out in less than two years given the time spent on the previous books (at least four and a half years for Winds, over five and a half for A Dance with Dragons and just over five years for A Feast for Crows) would seem unlikely. In a recent interview with Vanity Fair, George suggested that he might still be able to do it - although it would be a “tight,” - but this was in the same article where Benioff and Weiss indicated that eight seasons was still doable. Dropping it to seven would appear to make it almost impossible for Martin to stay ahead of the series without a dramatic (and unprecedented in this series) increase in writing speed.
Things could get awkward in early 2017. I could also see HBO pulling an AMC and stretching the final season into “two” seasons (which they’ve also done in the past with The Sopranos), to push the finale into 2018. But realistically, the final book probably still won’t be done even by then.