Brady clearly wants to play (at least he says he does) as long as he can. Practically speaking, it depends on the toll that injuries have/will have taken on him.
The experience of other HOF QB's says that he could have three or four more years playing at a high level, but that he also might see his skills decline sooner. The average retirement age of an SB-era, HOF QB is 37.5, usually, but not always, with a diminution of skills towards the end.
The only HOF QB to win a Super Bowl after the age of 35 was John Elway, who set the pleasant precedent of winning two at the ages of 37 and 38.
Brett Favre is the gold standard of performance by a QB into his late 30's and even after 40, but even he didn't win a championship late in his career.
Troy Aikman, taking care of his health in an enlightened way before today's emphasis on the risk of concussions, retired at 34, as did Joe Namath because of his ruined knees.
Terry Bradshaw and Bob Griese retired at 35. Dan Fouts and Jim Kelly at 36. Bart Starr and Roger Staubach at 37.
Elway, Montana, Tarkenton and Young retired at 38, but only Elway was still at the peak of his powers when he did.
The old timers were more likely to hang around longer: Dawson, Jurgensen and Unitas all retired at 40. Warren Moon was the outlier, retiring at 44.
