I'd wager that the show will overtake the books right around when the next book comes out. When that happens, I am almost certain that the show will continue on to the end of the story without the final book. GRRM has apparently already told the producers what happens, and certainly by that point, he'd know pretty well where it was going. They can't pause the story without having to recast, and who is to say that they'd even be able to get the funding together again three or four years later.
I've heard that this season goes through about 60% of the ASoS. The next season will almost certainly start getting into AFfC and ADwD--I'd imagine they will present those storylines interwoven and chronologically rather than splitting by character as the books do. Figure another season as well for AFfC and ADwD. This means that the first season for new material would be 2017, which I think (hope?) is doable for GRRM--it would mean TWoW would need to come out in 2016, which seems reasonable.