Half-Life: 13 Hours
Call of Doody Black Ops: 7 Hours
Call of Doody Ghosts: 5 1/2 Hours
Neverwinter nights: 54 1/2 Hours
Mass Effect: 17 1/2 Hours
Yeah, definitely not getting shorter.
R00fles!
Only last year's Ghosts was slightly shorter:
2007 Call of Duty 4: Modern Warfare 8h 05m
2008 Call of Duty: World at War 9h 33m
2009 Call of Duty: Modern Warfare 2 9h 13m
2010 Call of Duty: Black Ops 9h 30m
2011 Call of Duty: Modern Warfare 3 8h 06m
2012 Call of Duty: Black Ops II 8h 35m
2013 Call of Duty: Ghosts 6h 02m
Not seeing much of a trend with other major single-player developers:
2002 Neverwinter Nights 66h 43m
2003 Star Wars: Knights of the Old Republic 35h 08m
2005 Jade Empire 23h 18m
2007 Mass Effect 28h 29m
2008 Dragon Age: Origins 61h 04m
2010 Mass Effect 2 36h 17m
2011 Dragon Age 2 37h 59m
2012 Mass Effect 3 35h 43m
2007 Assassin's Creed 18h 35m
2009 Assassin's Creed II 27h 19m
2010 Assassin's Creed: Brotherhood 24h 50m
2011 Assassin's Creed: Revelations 19h 41m
2012 Assassin's Creed III 27h 12m
2013 Assassin's Creed IV: Black Flag 35h 42m
2014 Watch Dogs 28h 26m
2001 Grand Theft Auto III 24h 04m
2002 Grand Theft Auto: Vice City 32h 36m
2004 Grand Theft Auto: San Andreas 57h 48m
2008 Grand Theft Auto IV 38h 27m
2013 Grand Theft Auto V 42h 46m