What's the Most Time You've Spent with a Single Player Game?

In any game? I've played Zelda III and Super Mario World, and Star Fox 64 almost annually since release, so those, but I have no idea how many hours. I'm sure some arcade games or shmups would be up there as well, but no idea.

In one playthrough?

180-90 something hours in Xenoblade Chronicles X.

Demons' and Dark Souls are the only other 100+ I can clock in one playthrough.

FFXV (90+ saving post-game for after DLC releases) and BotW (@ 70+ as of this afternoon.)

Some of y'all are animals! I don't platinum or 100% games though, so maybe that's why? My multiplayer game clocks are pretty obnoxious though.
Skyrim: 657

2nd place for reference:
Dark Souls 1: 281 (400+ if you include PS3)

Skyrim clicked with me in a big way. Having only played F3 and NV before it from Bethesda's series, Skyrim just did everything better. And mods, of course.

Fallout 3 I wish I could give accurate numbers on, but most of the time was on PS3 or not tracked by Steam. I'd say at least 300 there.
I think I spent about 130 hours in Dragon's Dogma in total, which is by far the most time I've ever spent with a game. Next closest would be Demon's Souls with about 100 cumulative hours, and then after that would probably be Final Fantasy VII with I expect something like 80-90 hours across three playthroughs.
I've sunk far more time into the Elder Scrolls and Final Fantasy as series than maybe anything else hobby wise. I've purchased and bought most of the games more then once and played through them all multiple times, the way they work in general provides a lot of opportunity for replayability.

I'm honestly not sure if it would be a TES game or an FF game though.

If I were to include multiplayer games the easy answer would be Ultima Online.
In terms of just amount of time spent getting from start to finish in one playthrough, Breath of the Wild was about 115 hours for me, which is probably the most. I don't normally spend that much time on single-player games, at least not in a single playthrough. My previous highest time was 80 hours on FFXV.
According to Steam, I have 209 hours in Farming Simulator 17.
Before I built my PC in December, I put 90 hours into the PS4 version.

So 300 hours, and this is for sure the most I have played any game, and I have no signs of slowing down. Shits addicting.
Over the years, probably Doom - but i have no way of knowing.

I spent over 100 hours on FFXII confirmed, but that was dethroned recently by the 150 hours or so I've spent in NMS.

Probably doesn't sound much to some but it's a ton for me.
260 hours in Pokémon X
I think MGS5 is at 140
Fallout New Vegas is probably 100

Most of my time was spent playing Team Fortress 2 which is at like 350, I have friends who have played TF2 for like 3000 hrs.
I must have put, at least, 400 hours combined into Pokémon Red and Yellow back in the day. I would even say I got 500 easily. I put around 30 or so hours into them every few years, so the count keeps rising.

Diablo II: Lord of Destruction has to be around 400 as well. I played mostly solo, but did team up with a few friends from time to time.

Age of Empires II: The Conquerors has to be around 300 hours or so too. Mind you, I rarely, if ever, played multiplayer.

Diablo III nears the 200-250 hours easily as well.

Most recently, per Steam:

  1. OG Skyrim: 185 hours.
  2. Fallout: New Vegas: 140 hours.
  3. Clicker Heroes: 130 (around 129 idling, I guess)
  4. Dark Souls: 122 hours.
  5. Borderlands 2: 119 hours.
  6. Borderlands: 99 hours.
  7. Dark Souls 3: 98 hours (I still haven't touched the DLC).
  8. HITMAN: 93 hours.
  9. Mafia II: 86 hours.
  10. MGSV: 82 hours.

All of those in single player. I really don't like playing with others :p
Monster Hunter Freedom Unite- 500+ Hours
Edit: Well, I am an idiot. SINGLE PLAYER! In that case probably Double Dragon 2 The Revenge. 300 Hours? still counting since I still play it very often.
I posted above -50 hours on Skyrim (360)

Just booted up the PC and actually it's 442 hours on total war: Rome 2

That's scary how much I've played 1 game. That's enough of that!
Probably fallout 3 with something like 120hrs.

My guess is BOTW will surpass it. I don't know where I'm at right now but probably 60 hours or so after 2 dungeons and basically no side quests.
I'm at 80 hours in BOTW and I've only done 1 main dungeon... it's just wonderful being in that world. I've never played so much in such a short space of time before...
I'll never know for certain how much of my life was put into Resident Evil 4 or Final Fantasy VII, but I'm willing to guess it's easily 500 hours each. Even that seems low, knowing that I have close to 200 hours in The Evil Within between PS4 and PC, and I still don't know that game as well as the other two. Somehow.

The longest recorded total playtime I have available to me on Steam is Dragon Age: Origins at 109 hours. It seems laughably low by comparison.

But I'm also one of those people who only buys maybe 5-10 games a year and only really truly loves one or two of them per year, so take that for what you will.