Brady and Belichick have to be viewed separately when it comes to any decisions about retirement.
I think that Brady will play for as long as he feels he can contribute at a high level. It's the rare SB-era HOF QB who hangs around more than one or maybe two years after his skills begin to decline OR until the cumulative impact of the game on his body or brain becomes too great (the exceptions are Warren Moon and some of the old timers from the 1950's and 1960's, the latter who played in an era of lower salaries, lousy pensions and when few lucrative media or celebrity careers were available post retirement). The average retirement age for an SB era HOF QB is 37, and that includes some old timers and Moon who hung around into their 40's.
Given the pounding he's taken these last three years and thinking about the changes some purport to have observed in his performance after the Tuck hit last Sunday, I don't think he'll be too far off that average. He'll turn 35 this August, but, in QB years I personally don't think he's a "young 35." If I had to bet (and making it clear that I'm just guessing), I'd bet that he retires after the season that he starts when he is 37 or 38, the ages at which Montana and Elway retired respectively; Elway retired at the top of his game, Montana probably one or, arguably, two seasons after he began to decline.
So, that would give TB three or four more seasons, hopefully at least two of which will still be at the highest level. I really don't think we can kid ourselves any more that he is another Favre, who was never seriously injured and played at a high level until he was 38 or 39, before winding down for two final years in Minnesota. Way too much is understood now about the impact of multiple head and body traumas later in life for a guy with Brady's money and prospects/interests outside the game to hang around longer than it would be prudent for him to do so.
Belichick? I don't know. He is an intensely private person when it comes to what he thinks or how he feels about anything important, so none of us can know what is going on.
My personal gut feeling is that he will hang it up when he just plain gets tired of the mental and emotional grind and pounding. He looks like he's in pretty good physical shape, so, with some adjustments to his working hours, he could probably coach well into his 60's, which he turns this year. Tom Coughlin will be 66 this summer and he's still going strong and wants to keep coaching, which, clearly, nobody is about to say he shouldn't do.
