He said in the 2009 Football Life footage that he won't be coaching into his 70s like Marv Levy.
That gives him 5-6 more seasons coaching the Pats before that self-imposed deadline, whether he upholds it or not.
I also think it's worth noting that his son is now on the coaching staff. Belichick talked at length about how much that -- and their newfound father/son time -- meant to him in one of his press conferences this week, and I wouldn't underestimate the impact that could have.