Olivant's absolutely right: there is no way the Five Families war could have lasted more than two years (nor Michael survive in Sicily for more than two years); and that, as a practical matter, Michael could have afforded to wait until 1955 to whack the other Dons without the Corleones being completely marginalized. But, as Lilo said, it worked for the plotline of the film. That's one of the reasons why the films aren't a reliable source for at least a rough timeline.