Just started Dragon Warrior 7 for the first time and boy, I'm in it for the long haul. Considering the game is supposed to take atleast 100 hours to beat. 

What's the longest RPG you've ever played?

I`d probably say Fire Emblem Awakening. I could be on that game forever collecting all the dialogues. : )

Like the one that took the longest to beat or the one I've played for the longest?  I remember putting 150+ hrs into Pokemon Diamond, then I got fatigue on the series, still gotta beat White 2, X and AS.

I guess FFXII which I put 120 hours into and never completed. Still had Yazmat and Omega Mk XII as well as some summons and Rare Game... Come on HD remaster on PS4
Pokemon Diamond, over 1000 hours but thats across multiple save files.

Skyrim I would say, I remember putting a decent amount of time before being done with it. After that most likely Final Fantasy VII or IX.
Dragon Age Inquisition is getting close to that level.

FFX. It certainly felt the longest!

Fire Emblem Awakening for me. I've put almost 200 hours into the game and I'm nowhere close to having all the conversations unlocked.

But if we're talking about sheer game length then Tales of Symphonia takes around 80 hours and I finished my first playthrough at around the 75 hour mark.

I put 100+ hours into pokemon Emerald after all the traveling in cars that my family did as I was growing up. Star Ocean Till the End of Time also kept me going for 100+ hours, I have no idea how I managed to do that.

Oblvion, Oblivion had significantly more to do than Skyrim.

hint: "gotta catch 'em all"