Not sure how long it took me since the game was released in four installments over a year and change but .hack series is the longest rpg I've played. Played all four parts. Infection, Mutation, Outbreak, and Quarantine.

If counting just one game; Pokemon Diamond with 2200 hours registered. Pokemon Black came soon after with 1500. I understand these numbers may be hard to believe, but keep in mind that I brought both during launch day and played both extensively. Diamond took longer because Platinum nor HeartGold/SoulSilver peaked my interest that much; as did Black 2/White 2. 

150hrs in Dragon Age inquisition so far. Had to take a break from the game. Im too much of a completionist.

Mmorpg - Final Fantasy Xi with around 7200 hours otherwise rings of power on the sega megadrive. Seriously play that without the walk through. That game kills you just for the hell of it. Set up camp to revive some health or to miss the creatures that come out at night and you may encounter the message- by some cruel twist or fate you are dead.

Persona 3 Portable. I think I hit ~70 hours for my first playthrough. I'll likely go over 100 hours once I do a NG+. Persona 4 Golden will definitely go over 100 hours, too. Problem is that I've been busy with schoolwork.

Xenoblade Chronicles. My original save file was 125 hours. I played through NG+ to experience the story again immediately, putting it up to about 150. And now, a couple years later, I'm just about done with a 2nd, near-100% completion play-through that's at about 140. And I've had a couple other play-throughs where I got about 30-40 hours in. So I've put somewhere between 300-400 hours into the game, and in a couple months I'll be playing through it again on 3DS! :D That game feels like "home" to me at this point.

I've also put about 140 hours into Fire Emblem: Awakening through 2 play-throughs.

I managed to beat FFXII in about 80-90 hours but I didn't do most of the hunts or even challenge Yazmat. If I were to do all that, then my playtime would definitely exceed 100. Oh boy, FFXII was epic. Here's hoping for an FFXII HD.

Star Ocean Till the end of time. Level up to 255 then spend forever preparing to take on Freya; who has something like 20 million HP normal.

I just watch them on YouTube, saves me the time xP

Both. Although, I think you're answer will be Legend of Dragoon or a Disgaea game.