It seems like the the natural limit is between 10 and 15 years, US president Franklin D. Roosevelt at 12, UK Prime ministers Margaret Thatcher and Tony Blair at 11 and 10 respectively and Spanish PM Felipe González for 14 years.
Have any democratic leaders exceeded these amounts of time as leaders of their nation (either consecutively or non-consecutively)?
What enabled them to govern democratically for so long, whilst other leaders fall by the way side?
The top level of the Swiss Government is made up by the Federal Council, a body of seven members hailing from various political parties. Their serving time is technically unlimited (though has to be renewed every so often). There have been several Members that have served for more than two decades, the record being held by Karl Schenk, who was a member of the Council for 31 years.
Canadian Prime Minister William Lyon Mackenzie King served a total of about 21-1/2 years, over three periods:
(Excel tells me that those periods add up to 7,826 days, which is about 21.4 years.)
He's also featured on our $50 bill.
Swedish prime minister Tage Erlander was prime minister for 23 years from 1946 to 1969. During this time he "won" seven election with his party gaining over 50 % in the last election before his resignation.
I know this is not as head of Government but Eamonn de Valera was President of the Irish Executive Council for 5 years until 1937. the he wrote a new constitution making the president a figurehead and became Prime Minister for 16 years.
afterwards he retired to be president for 14 years.
So he was head of the government for 21 years ( not consecutively) and figurehead for 14 until 1973.
Not bad for the man who "lost" the civil war.
Just how democratic post independence Singapore is is up for debate, but Lee Kwan Yew served as pm from independence in 1959 to 1990. He then held various ministerial positions until 2011. The party he founded has been in office since independence, and his son is the current pm.
The first prime minister of India after independence, Jawaharlal Nehru was prime minister from 1947 until his death in 1964. He contested four elections, 1947, 1952, 1957 and 1962 and his party won majority (albeit diminishing) in all four of them.
Someone else here could answer the 'why' authoritatively.
The eighth president of Finland, Urho Kaleva Kekkonen served continouosly from 1956 to 1982 - 26 years in total. His long - and by the end increasingly autocratic - rule was partially due to his shrewd management of the delicate Finno-Soviet relations during the era of Finlandization. Several amendments were later made to the Finnish Constitution to move power from the president to the ministers and parliament.