"[The number of seasons] is being discussed as we speak," said Vogler, via The Radio Times. "The third season was the first half of book three, season four will be the second part of book three. George R.R. Martin has written books four and five; six and seven are pending. I would hope that, if we all survive, and if the audience stays with us we’ll probably get through to seven seasons."
At this point in time, there's no indication of a release date for The Winds of Winter, Martin's sixth book in the A Song of Ice and Fire series, so assuming the show does run for seven seasons, it's looking increasingly likely that the series will catch up to the books and have to deviate from its source material.
Game of Thrones season 3 is currently airing on HBO and Sky Atlantic.