On all formats, I think the longest playing times are effectively:

THE ELDER SCROLLS V: SKYRIM
According to the hint book, it will take you about 800 hours to play the game in every permutation possible. In practice, it takes about 40 hours to complete the main quest and a chunk of the side-quests (where I am now), and probably 100-120 hours to do everything it's possible to do in the game with a single character. FALLOUT 3, NEW VEGAS and OBLIVION fall into the same bracket.

BALDUR'S GATE II: SHADOWS OF AMN
Along with its expansion, THRONE OF BHAAL, BG2 takes about 100-120 hours to complete, or possibly up to 200 if you try to do everything that's possible in the game with one character. BG2's permutations, particularly of interparty relationships, are far more complex than even SKYRIM's, offering much greater replay value.

THE MASS EFFECT TRILOGY
Played as one long mega-game, the entire thing should take you about 90 hours to play. Individually the games are actually pretty short, even for big RPGs (25-30 hours).

If we want to be really cheesy, there's THE ELDER SCROLLS II: DAGGERFALL, where visiting every dungeon and exploring the whole surface map will take you several thousand hours. This is a cheat, though, as the game is mostly procedurally generated (i.e. randomly created through programming tricks, not hand-designed).

As for shortest, that's more difficult. I play primarily single-player, so pretty much all of the CALL OF DUTY games (especially the 4th and later games) are pretty feeble. Other than that, I usually find 8-10 hours to be the minimum I expect to get out of a game, and it's rare to find one that is less than that. MAX PAYNE 2 is the most notable example (clocking in at 5 hours or so), but that gets a pass for being so utterly excellent from start to finish that it makes up for the short length (the same thing for PORTAL).