My ruff estimates:
Oblivion - 2000 hours.
Age of Empires II - 1500 hours
Dark Age of Camelot - 1500 hours
Warlords II - 1000 hours
Lords of the Realm II - 3-400 hours
Return to Castle Wolfenstein - 3-400 hours
Enemy Territory: Quake Wars - over 250 hours
Warlords - over 200 hours
Morrowind - 200 hours
Quake II - 200 hours
Bad Company 2 - 200 hours
Fallout 3, Dragon Age, Heroes of Might & Magic and others - around 100 hours each







