Just guessing, but I'd say Belichick will hang in there for another 4 years.
Agree that he may not want to be coaching in his seventies. It'll be interesting to see what BB DOES do upon retirement though. He loves every aspect of the game from assessing college prospects to training young coaches etc etc. I don't expect to see him sitting behind a desk at ESPN! Anyway, I suspect BB will remain for a little while after TB retires and he'll be challenged by the task of reconfiguring the team under another QB.
Tom may have another 3-4 years left in in him, but I'm going to go out on a limb and say he'll play TWO more seasons. His wife can be expected to continue trying to persuade him to retire and his children may begin to exert a greater claim on him and his time. What I can't envision is what TB would DO in retirement. I don't think he'd be content with simply a jet-set life of play. Would his fitness program and products --the whole TB-12 thing he is doing with Guerrero-- be enough to engage him? I dunno.
I will say I have known numerous people who remained in their conventional careers overlong because they simply didn't have a vision of an appealing retirement activity.