De Gaulle (1959-1969): 10 years
Mitterrand (1981-1995): 14 years: don't see anyone coming close to breaking this record. Due to health problems, Mitterrand chose not to seek reelection to 3rd term in 1995.
Chirac (1995-2007): 12 years: 2nd longest-serving French President. Didn't seek reelection...