For me it would have to be Fallout 3 on the Xbox 360. When I first got it after seeing my friend play it I couldn't stop. I would stay up all night for DLC to come out at like 6 am EST, I was there on release day for all 5 DLC. When my xbox failed and I lost all of my saves and data I started getting into game save mods, So many wasted hours.
So if I had to guess, I would say over 200 hours in total.
On PC it's probably Killing Floor, spent over 300h in that game. On PS3, I've spent over 300-400 in UC3, I think at least.
I've also spent 118h playing Dark Souls on PC and over 200h on PS3.
I got Fallout New Vegas Ultimate Edition from Steam for 5€, so I'm definitely gonna spend lot of time on that one. I've never finished any of the DLCs except Dead Money.
There are multiple games where I've played on more than one occasion, where the games never end. Animal Crossing, I'm look at you. But, if you're only looking for completion times then I would say that Final Fantasy XII springs to mind as one of the lengthiest games I've racked up several hours in. I think by looking at the clock, I had invested about 85 or 90+ hours in the game, much of which came from monster hunts alone.
Of course, there are other RPGs out there where I've beaten the same game on more than one occasion such as the original Chrono Trigger or Final Fantasy VIII. The Elder Scrolls IV: Oblivion took two months to complete the game; about the completion time I couldn't tell you how long the game lasted.