I've been keeping track recently of the time I spend in each game. For older games (before September 2007), these are approximate times.
Excite Truck - Approximately 10 hours to the end of the 'Normal' mode
Gears of War - Approximately 10 hours solo medium campain, 12 hours coop insane campain
Half Life 2 - Approximately 20 hours
Heavenly Sword - 7.5 hours normal mode
Marvel Ultimate Alliance (360) - Approximately 10 hours
New Super Mario Bros - Approximately 10 hours
Pokemon Pearl - 70 hours to being able to easily farm Elite Four
Portal - 4 hours, no advanced maps
Puzzle Quest - Approximately 50 hours
Ratchet and Clank Future: Tools of Destruction - 12 hours normal mode
Resistance: Fall of Man - Approximately 13.5 hours coop medium difficulty
I created an Access Database that tracks time spent in each game and hardware and software purchases. From this, it will output Sum of Hours by game and platform. I'm still trying to add more features.
If anyone is curious about the database, PM me, I'll send it to you, you can edit the fields as you see fit.