I got curious about the longest games of Diplomacy played, so I decided to look for them.
A quick search reveals that the longest game on vDip lasted 57 years. It was a WW4 game where Germany soloed with a 100-SC target:
http://vdiplomacy.net/board.php?
gameID=30000#gamePanel
While the longest game on WebDip has lasted 284 years and is still going:
http://webdiplomacy.net/board.php?
gameID=166960#gamePanel
And the longest game on WebDip that appears to be legitimately serious lasted for 105 years:
http://webdiplomacy.net/board.php?
gameID=93086#gamePanel