had the same rulers/ruling party for decadesAlternatively (with apologies for playing a bit fast and loose in terms of President/Prime Minister/Secretary - political systems come in many shapes and sizes) and note that in some of these political party doesn't make much sense - when a military strongman stages a coup and holds power for a decade and a half, is there really a ruling political party?
Japan-
Liberal Democratic Party: 1955-1993, 1994-2009, 2012 to present
Singapore-
People's Action Party: 1959 to present
Lee Kuan Yew: 1959 to 1990
Goh Chok Tong: 1990 to 2004
South Korea-
Syngman Rhee: 1948 to 1960 (interestingly, exempted himself from an eight-year limit in 1954)
Park Chung-hee: 1963 to 1979 (took power in military coup, assassinated)
Taiwan-
Chiang Kai-shek: 1950 to 1975
Kuomintang Party: 1950 to 1996, had the advantage of being a single-party state until roughly late eighties
Definitely some decades in there.