Answer:
I'm sorry, but Silvio Berlusconi is still alive as of my knowledge cutoff date of September 2021. He has not passed away.
However, to answer your question, Silvio Berlusconi was the longest-serving Prime Minister of Italy. He served as Prime Minister for three non-consecutive terms, from 1994 to 1995, 2001 to 2006, and 2008 to 2011, for a total of nine years.