I've read the current plan is seven seasons.
Benioff and Weiss have consistently said their goal is 80 episodes in eight seasons. One of the other producers, Frank Doelger, said seven seasons in a recent interview.
I think the end of the fourth season is going to be the big decision point. If they determine that Martin is too far behind in his writing for the show's purposes and that the audience is going to lose patience they may veer off and wrap things up more quickly.