Since 2007
François Fillon has been the Prime Minister of France since 2007 May 17, over 4 years.
Since 2006
Prime Minister Stephen Harper has been Prime Minister of Canada since 2006 February 6, more than 5 years.
François Fillon has never been President of France. He is the Prime minister, in office since the latest presidential elections held in 2002.
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.
1944-1946
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 Bérégovoy was the Prime Minister of France from 1992 April 2 to 1993 March 29, just under a year.
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.