As a comparison, Muhammad Ali, held his title--World Heavyweight Title, and later, the WBA Heavyweight Title--for four years, between '64 and '67. He didn't fight for the next three years because of legal troubles. He came back in late '70 to win the NABF title, a title that was specifially created for his return to the ring. He contended for the WBA and WBC Heavyweight Title held by Joe Frazier in'71, and lost.
In all, Ali won 56 fights, 37 by KO, and lost 5.
Mike Tyson held the Heavyweight Title for four years also, begining in '86 with the WBC Heavyweight Title. In early '87 he unified the WBC and WBA Heavyweight Titles in his win against James "Bonecrusher" Smith. In late '87 he unified the WBC, WBA, and the IBC Heavyweight Titles in his win against Tony Tucker. He held all three titles until '90 when he lost against James "Buster" Douglas. He came back in March, '96 to win the WBC Heavyweight Title agains Frank Bruno, and lost it later that same year against Evander Holyfield.
In all, Tyson won 50 fights, 40 by KO, and lost 6.
Between the two, I think Ali was the better boxer.
Lennox Lewis held his titles longer, but not as consistently.
http://www.boxrec.com/boxer_display.php?boxer_id=000180
http://www.boxrec.com/boxer_display.php?boxer_id=000474
http://www.boxrec.com/boxer_display.php?boxer_id=001853