Before Mamberti...

... interesting to look at some of his most recent predecessors, and the length of their terms as Prefect of the Supreme Tribunal of the Apostolic Signatura
Most informative.
Ten years seems tops, (and those longer tenures were during years of unprecedented ecclesiastical upheaval,) unless I misread, and six  is longer than most. 

• Giuseppe Bruno (20 Mar 1954 - 10 Nov 1954)
• Gaetano Cicognani (18 Nov 1954 - 5 Feb 1962)
• Francesco Roberti (14 Nov 1959 - 24 Mar 1969)
• Dino Staffa (7 Apr 1967 - 7 Aug 1977)
• Pericle Felici (13 Sep 1977 - 22 Mar 1982)
• Aurelio Sabattani (17 May 1982 - 1 Jul 1988)
• Achille Silvestrini (1 Jul 1988 - 24 May 1991)
• Gilberto Agustoni (2 Apr 1992 - 5 Oct 1998)
• Zenon Grocholewski (5 Oct 1998 - 15 Nov 1999)
• Mario Francesco Pompedda (15 Nov 1999 - 27 May 2004)
• Agostino Vallini (27 May 2004 - 27 June 2008)
• Raymond Leo Burke (27 June 2008 - 8 Nov 2014)
• Dominique Mamberti (8 Nov 2014 - )