Considering the ridiculously good shape he stays in and the fact he doesn't have much of a history of injury, I see Chara playing at a high level for another 4 or 5 years. If he wanted to keep playing beyond those 4 or 5 years, I wouldn't be shocked to see him playing for a few years even beyond that. He's so big and strong that he doesn't have to play a brand of hockey where he takes things to the limit physically. He actually holds back quite a bit when he hits so that he doesn't draw as many penalties, plus he can be effective even when he doesn't hit with full force.
As long as he remains a solid shutdown defenseman, he could stay on the top pair with Hamilton for a lot of years. At minimum, playing out his contract, which goes until he's 42.
Hamilton will likely take over a lot of the "#1 defenseman burdens" in the coming years. If he has a sophomore breakout, which honestly seems pretty likely from the way he's played his rookie year, Chara and him could be paired as soon as the start of next season. He'll still be one of the best defenseman in the league for a couple of years for sure, and he's literally the perfect mentor from him. Hamilton's offensive talents are off the charts good, and they'll only get better with time and added responsibility, but Chara can teach him so much defensively. A very solid two-way player in Hamilton and a defensive rock in Chara could be a terrifying tandem for a good couple of years. His conditioning is insane, he'll only stop playing when he wants to stop playing. I doubt he'll want to retire as a pylon, but when he wants to retire is completely up to him. His body won't likely be the cause.
It will come down to desire and salary. Physically he could play for 10 more years (barring injury of course). Will Chara want to play a diminished role and will he accept a reduced salary in order to keep playing? He will play out this contract and then I'm guessing his future will rest on what offers he gets.
I think two more years as a top pairing dman, then another 2 years as a depth dman on a cup contender.
This isn't a video game, his overall wont drop down to 80. As long as Chara is playing, he'll be getting big minutes on the ice. He'll probably play out his current contract, barring injury, then retire.