We're left with an embittered and alone Michael Corleone in 1968 at the end of II
I think it was 1960 or '61 at the latest, if we take the cue of Roth returning to the US to vote in the presidential election (1960). Neither he, nor Rocco (when he assassinated Roth, nor Michael brooding in the boathouse, looked eight years older than Roth did in Havana, and Michael and Rocco did in the penultimate boathouse scene where Michael has his kill-'em-all-and-humiliate-Tom scene.
I believe I read in the original shooting script, the final scene was supposed to be set in 1968 with Anthony denouncing Michael, and leading to what we do see, and that they were going to film it, but lost the light. If you look at Michael in that final scene you'll see his hair is shorter and a little differently styled than in the 1958-1960 segments, little graying added, and more wrinkles around the eyes than in the scenes leading up to Roth's assassination.
Per IMDB:
"According to the script, the movie's last shot in the film centering on Michael Corleone as he gazes at the lake, occurs in 1968. That accounts for Al Pacino's additional wrinkles and slightly receded and greying hairline. It was actually the concluding aspect of a scene with his son, Anthony Corleone, who declares that he will not follow in his father's footsteps. Anthony was portrayed by an actor about eighteen years old; the scene was halfly filmed, but Francis Ford Coppola lost the light before wrapping for the day, and was unable to return to complete the scene."
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