If I remember correctly the events of the novel and movies start in late 1945 (with flashbacks obviously). By the time Michael makes his move against the other families, he's been married to Kay long enough that their son is already reading (remember he jokes with the Don about the funny papers); so it has to be at least the early 1950s. He kills the police captain in early 1946, hides out in Sicily, comes back, waits a year before he even contacts Kay--so they're not married until at least 1948.
Sounds about right. I still wish that we were filled in on how Michael amassed such an empire by the time Godfather 3 comes around. I’m assuming it’s all from real estate, casinos, and illegal activities but he made enough legitimate money to sell off the illegal portion of his businesses and live comfortably as a man worth 9 figures.
