No my longest but Valkyria Chronicles 3 took me about 130 hours to finish. I have played many FF or pokemon games far longer than that but that was well beyond the main story.

Diablo II + LOD

Before I reinstalled the game last year, I had made about 15 different level 90 characters...all of them offline (so no Hell Baal boosting). Also, thanks to the in-game item management mod, I farmed for all the uniques and set items in the entire game, again Offline.

I estimate about 2500 - 3000 hours in total .

Really? It was my favorite of the 3. I played with every class, especially the soldier. To me that game has a lot of replay value. In some sense 3 is better - others worse - but it didn't sway me as much as 2. The gameplay was more refined and I liked that, but I found 2 to be more challenging, so I stuck with it. I blazed through 3, even on insanity. Which system did you play it for? It was great on PS3 with all the DLC on disc with the excepion of Arrival (I think that's the name of it).

Verifiable:
FF12: 127 Hours
Most FF's: 50-60 Hours
Fallout 3 (including expansions): 50 Hours

Star Ocean: Last Hope- 52 Hours

Unverifiable:
Diablo 2: Can't Imagine
Runescape: Can't imagine
Pokemon Blue/Yellow/etc: Can't imagine. I always used gameshark though so I caught them all and levelled fast

Objectively speaking, Xenoblade has the longest campaign of any RPG I've played.

But I'm pushing 200 hours on Fire Emblem: Awakening, and I have no clue how much time I've spent playing The World Ends With You.

Difficult question.

I haven't played a whole lot of RPG's and most I never finished. For example I've played Oblivion for maybe only about 10 hours, and Fallout 3 for about 40 hours. The biggest ones I have actually finished are Pokémon Blue, and Star Wars Knights of the Old Republic.

Don't really know how long I've played Pokémon, but I know it was looong. Could very well be up to something like 500 hours.
I'd give Star Wars Knights of the Old Republic the win though, because I actually know a real number, but only because I did many play-throughs. I think the game's brilliant, so I've played through it five times. On average, a play-through would take me 50 hours, some were 60, some were 40. All-in-all I'd say about 250 hours total for SWKotOR.

multiple playthroughs:

dragon age origin

single playthrough:

skyrim

Pokémon Emerald. Hell, I've played that game a million of times, I can't even count how many hours I spent playing that(and every other Pokémon release for the GBA).

Pokemon Heart Gold

136 hours and still haven't beaten Red yet. Still grinding on the Elite Four.

Persona 4: Golden

83 hours to complete the game. Incredible considering how fast paced and story intensive the game is.

Fallout 3, about 60 hours.