I think definitely with a loss or maybe a loss followed by final homecoming fight against a softer opponent, or something like that. As long as he keeps winning he'll keep fighting.
I think he'll win a couple more fights (and surprise some people in the process), lose a couple more (getting beat decisively for the first time since 1993), and finally go out with a win over a lesser opponent in Philadelphia or Atlantic City around the age of 50.
I could see him losing a couple and winning a couple over the next 4 years or so then retiring. I think now or after he (IF he) beats dawson would be a great time to retire
Fifty years from now, in the midst of his 187th professional match-up, Hopkins is matched with a fresh new 28 year old undefeated contender. At that point I see the old lion heroically fighting just a little too slow and just enough out of sync to falter before absorbing a tremendous, beautifully thrown lead right cross, followed by a rapid three hook combination to the solar ribs, and a final devastating Tyson-like right uppercut landing perfectly with such force snapping Hopkin's head far back beyond what one would think was the natural range, before the old man gives up, crumbles, and is carried off on his shield. The year would be, 2066.
I see him losing before he retires in at least one of the big fights he'll take, although he'll probably finish it off with a "W" in a meaningless last goodbye fight.
America wants dawson to win the belt and then defend it against bute. if the fight between dawson and hopkins is close, dawson will win. the hopes for the promoters who run boxing are that dawson wins and acquires a lot of Hopkins' fans and then they can make dawson vs. bute a mega fight since dawson has a more long-term profit than BHOP, who has been old for 10 years now. being screwed out of another close decsion will discourage Hopkins, and he will either go up in weight to fight either the crusierweight champ, or something even more crazy like heavyweight to quel his ambitions and satisfy himself enough to allow his retirement.