But it is very common that Italian mafia members can wait years, even decades before seeking their revenge. That's because of the honor they have in their society. A mafioso never forgets. So imo it is quite plausible that Michael took his revenge years later.

But I understand you're point, you're right about that it may not be realistic if a mob war takes so many years. That would probably ruin their business. The real life Castellammarese War also lasted about two years.

So I think the Five Families War in the Godfather lasted from December 1945 till August 1947, when Sonny was killed.

Btw, didn't someone at the board mentioned the cars at the scene in which Kay and Mike reunited where from 1948?