This depends on which Pierre is being spoken of. See related questions.
Since 2006
Since 2007
Pierre Bérégovoy was the Prime Minister of France from 1992 April 2 to 1993 March 29, just under a year.
Prime Minister Stephen Harper has been Prime Minister of Canada since 2006 February 6, more than 5 years.
The current Prime Minister, Recep Tayyip Erdogan, has been in power since 2002. In Turkey, a Prime Minister can remain Prime Minister as long as his party or coalition maintains over 50% keeps nominating him as the Prime Minister.
There has been no such Prime Minister of Japan. You may be thinking of Taro Aso.
Hun Sen has been Prime Minister of Cambodia since 1988 November 30, almost 23 years.
Pierre Trudeau was Prime Minister on different occasions, non consecutively, for a period of about 17 years.
30 years
3 YEARS
No British Prime Minister has been in office for anything LIKE this long- the longest serving Prime Minister to date has been Lord Liverpool, who was in office from 1812 to '27, a total of 15 years.
Dean Barrow has been the Prime Minister of Belize since 2008 February 8, over 3 years.