Not counting endless games like MMORPGs (World of Warcraft) and sandbox games (Oblivion, Skyrim, etc.) because that's not really fair... well, here's the some of the longest ones I can think of for completion time:
Dragon Age: Origins - 45 hours
Many have said DA:O is a 50-60+ hour game for completionists, but my fairly typical first playthrough lasted a respectable 45. This doesn't included the expansion either, largely because I never got around to finishing that for reasons I cannot remember.
Dragon Age II - 44 hours
Interestingly, DA2 actually ended up not being THAT short a game. The endlessly recycled environments and over-abundance of enemies coming at you in waves weren't it's strong points, but there's not an excessive amount of padding in that completion time. Unlike DA:O however, this is closer to a completionist run; very little was passed over in DA2, while there's certainly quite a few things I didn't do in DA:O.
Final Fantasy X - 88 hours
Using FFX here mostly because I can actually remember the time I have on the save file, and it's from a completionist run of the game (worked on clearing the monster arena). I think that I have a save file ready to tackle the final boss at around 65 hours, so the extra 20+ is coming from trying to complete the monster arena and other sidequests. As for other titles in the Final Fantasy series that I can recall? I think I have a finished FFIX game with ~50 hours to defeat the final boss.
The Legend of Zelda: Twilight Princess - 88 hours
After three RPGs, a genre generally known for having longer games than the norm, an action-adventure title comes in to defeat two of them handily (and neck-and-neck with the third). Time just flies by in this game, and a recent (partial) playthrough had me noticing that the first six or so hours of the game is barely the beginning (just reaching the second dungeon)... and it didn't feel like it took anywhere near that long. No real padding either, just quality gameplay.
There could be some other potential candidates, but these are the ones with times I can actually remember.