Frankly it kind of seems most of the cast is getting tired of the show. They are actually all incredibly talented, more than a lot of teen show's actors are and the lackluster writing is probably frustrating them.
Penn even made a comment at Sundance about being cast in his movie there where he was offered the role and he was like "really? have you seen Gossip Girl?"
I'd say they have one more year in them. Or whenever their contracts are up, I doubt the actors will renew.