I can never find myself playing the same game for days and days. The most I've played a game for was about 50 hours. That was Skyrim and Fallout 3. Borderlands (1) couldn't have been too far off that either, by the time I had finished all of the DLCs.
Online: Cod4 (35 online days) and World at war(30 online days+ many days in zombies)...
Offline: Red Dead Redmption and GTA4 10+ days in single player only on each.