I tried to tackle this subject a long time ago by using the novel as a timeline guide. My conclusion was that Michael was in Sicily for far less than two years, and that probably less than three months elapsed between his being hit by the thunderbolt, and Apollonia's death by car bomb.

http://www.gangsterbb.net/threads/ubbthreads.php?ubb=showflat&Number=580389&Searchpage=1&Main=20650&Words=%2BMichael+%2Bin+%2BSicily&Search=true#Post580389

Only Sr. Vitelli knew that he was hiding out, but the entire village knew that an American with family ties to Corleone was among them because all of them were there for the wedding. From then, it was just a matter of (brief) time before word got back to Barzini, and Fabrizio was recruited to kill him.