He's heading for 40 now but luckily there's no surge of young heavyweights so he's probably good for a few more years.
His style is not based on the elements that go first, such as quickness or reflexes, but eventually he will slow down so that people start catching him with quick attacks. However I assume he will retire before that, maybe in a year or two max. simply because he doesn't have much to do in the division and his body is giving out
I know he was quite injury-prone but I think you're right. Probably another 2 years or so. Or until he gets bored.
He hasn't been busy, hasn't taken too many shots and importantly, he knows how to train properly, ie like a modern athlete. He isn't running 5 miles every day depleting his system & ageing himself like the majority of historical fighters have, so he should be able to last much longer than most. I reckon he's good for another 4 years unless somebody decent comes along.
I don't think he'll stay all that much longer, myself. That's just a hunch, mind, but he is nearly forty, & has that long list of both domination & injuries --- two pretty solid reasons to hang 'em up in the near future. There really isn't anyone good enough right now to give him such a beating as to convince him he can't do this anymore, but I don't see him as the foolhardy type (ala 99% of all boxers) when it comes to his reach exceeding his grasp. He's not delusional the way most fighters on top (or near it) are. I think one, perhaps two more years. His brother will likely exit not all that far behind him, either.
Had he not taken a four-year break, it's possible that he'd be done by now, or within 1-2 fights. But allowing his body to properly heal during that time greatly benefits him, especially since - as already mentioned - there's not exactly a looming threat out there to send him into retirement anytime soon.