Most of the times listed are estimations:
- Diablo II: Lord of Destruction
At around 6 years (give or take a few months).
I bought it in mid summer 2001, played pretty much every day of the week, and every week-ends with very few exceptions for a solid 2 years; then still played very regularly with only a bit more spacing between sessions for another 4 years or so (about 1 year and a half played offline, then moved to B.net for the rest until I finally quit). This case is particular for me. This is the only game in my own history of being a gamer (since around age 9 up to this day) that legit negatively affected my social life and my physical health. I called in 'sick' at work more times than anyone should have even if they had a passionate hobby (thankfully I was done with school by then, even though it was still relatively recent).
It's also the only game ever that forced me to act against it to help myself get out of my addiction. I simply looked at the disc one day (LoD's) and snapped it in two and threw it in the garbage bin. So I don't know the number of actual played hours, at all. I wouldn't want to know. The real answer is: too much. It's my personal record to this day I believe. I did play other games (listed below) a good amount, sure; but never again like I played D2 LoD.
- Allods Online
At around 2 years (give or take a few months).
That's the first MMORPG I invested myself into seriously, time and money-wise. I had heard of and tried a few other MMOs prior to Allods but at the time the very concept of "MMORPGs" was still relatively recent for me, even though it had been around for a long time already; it just wasn't a genre in gaming that I had payed much attention to on an individual basis. If my memory serves me right I started to play it around summer 2010 (can't recall exactly what brought me to the game but I do remember it being, or "going" F2P in an update that had been advertised to some extent and I probably just happened to read about it, it was free and I had nothing else to play maybe at the time; so I gave it a try).
I played it also very regularly, joining guilds (not many though), doing guild activities, doing end-game raids and such; typical 'engaged' and dedicated MMORPG player who had no life at that time (I was unemployed then, and wasn't looking for work that much either; it was a weird period in my life too but I'll skip those useless details). I played it very regularly for about one and a half year, then I started to slow down my play frequencies and session duration. I would let the guild leader know ahead of time I would not go to certain raids or guild fights and I almost stopped random grinding in end-game zones. The gameplay itself started to get boring and the blatant P2W iron fist that the game was at its core eventually served as one of the big final nails in the coffin for me. Among other reasons, but also including bad decisions from the devs (in my book anyway) with overall just bad updates made me quit entirely.
I do remember quitting in early 2012 (around February I think) and took the opportunity to sell my account (which I had spent about $400 on, which itself was absolutely nothing compared to some individuals who were known at the time in specific "high-end" guilds to have spent absurd amounts of money going well into the 30K to 40K and no I'm not exaggerating).
- TERA
At around 3 years (give or take a few months).
Some time then had passed since my venture into MMORPG territory and between Allods and Tera I had only played a little bit of Guild Wars 2 if I recall correctly. However, GW2 didn't ultimately hook me enough to stay (to this day I still own the game but I have never made it to character level cap a single time; my highest level was about 68 or 69 which if I remember is literally 1 or 2 levels away from actual cap). So one day I saw previews and reviews of that new "Tera Online" game, which at the time was on a subscription model but a short week-end long 'open beta' popped in.
So I downloaded it to give it a try and was absolutely blown away by the physics and dynamics-based real time combat system. It was such a day and night difference between it and Allods' archaic tab-targetting with auto-aiming enemies and constantly lock-on to your movements that I absolutely had to give it a try. And, on the plus side, the game was genuinely very pretty to look at (those Koreans have their way with scenery, architecture, character designs and styles). And the soundtrack was in large part composed by none other than Inon Zur (also Dragon Age composer; and on a side note Tera's OST has a bunch of tracks that absolutely have that Dragon Age structure and 'flair' to it).
I genuinely really enjoyed the first year and a half or so (even more than Allods, actually way more so). However, by around the second year kicked in the game unmistakably started to change for the worse (gameplay wise, dungeon design wise and so on), and its "dress up simulator" nature and silliness personally pissed me off more than anything else. The game had legit bad ass costume designs originally for all races with some sexy-yet-universe-fitting ones for those who liked to see a bit more skin on their toons; all of it was in good taste with great artistic design choices. But after a certain threshold of tolerance I couldn't stop ignoring those otherwise awesome characters running around cosplaying neon-bright animals and using barbecue items for weapons with a grill for a shield.
But past the stupidity for aesthetics the gameplay started to suffer as well (especially end-game) and the severe drop in general difficulty to "alleviate" leveling up, along with actual removal of content (rather than to improve it) acted as final punches to my gut. I had to quit, and did. That time around, however, I didn't sell my account (harder and less common to do for that game, was still done though but I didn't bother to; and contrarily to Allods I didn't spend as much on that one anyway).
- Mass Effect 3 Multiplayer
At around 1,600 hours.
I could check on my Origins account but I still remember the number fairly well. I hated the campaign, but genuinely really loved its multiplayer. I played it daily for months, probably more than a year actually. I still play it from time to time to this day (maybe once or twice a month). And there's not much to talk about here really. It's just ME3's MP. I can only say that I didn't think (really) that Andromeda's MP would (or could) be worse. But it was. Still to this day I think that ME3 MP has been one of the best cooperation-based online action-focused game I've played.
It was simple, didn't have too many layers of complexity, wasn't P2W (literally never spent a single penny on it and unlocked absolutely everything there was to unlock and leveled up everything that could be leveled up) was overall well balanced and despite some technical and A.I. issues here and there was rather polished for what it was in the end (just a "branch" off the campaign that served more of an experiment for the franchise at that point than anything else). Just loved it. I still like it today and would still recommend it. The players base has of course massively dropped but it's still technically active and possible to find lobbies and people.
- Team Fortress 2
At around 1,500 hours (give or take a few dozens I suppose).
It's similar in time played to ME3 MP and a bunch of other online multiplayer-focused games I dedicate a nice chunk of time for (others would be the likes of Quake 3 Arena on the Dreamcast, UT99 on PS2, UT2004 on PC, Left 4 Dead; and a few more). But I'm specifically listing TF2 because at the time when I got it (Orange Box during its week of release, still have the retail box too) I thought it was the absolute best multiplayer game I had ever played in my life. And to honest, it was for some time too. It was a genuine gem of a game with superb Pixar-like art style like I had never seen before (especially not for an online game). It was just charming and its characters were memorable (it helped to give them each a personality with those short CGI presentations; "CGI Shorts" in the veins of Blizzard's Shorts for Overwatch before that stuff came, Valve at the time were probably ahead of the curve).
But yeah, it's TF2, everyone knows what it is so there's not much else to add here. I did stop playing it entirely but not because I "hated" it at any point. I just had my fill and moved on, but I have no regrets; absolutely loved my time playing it.
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Now I could go on and on. There's of course the hundreds of single player games I played a lot of from 8-bit years and platforms to now, but I picked the ones above since those really have been cases where I was literally doing marathon sessions on a regular basis. I mean if I accumulate all my time ever that I have played the Super Mario Bros games it would probably amount to a solid thousand hours or even more across all the years and various games but, indeed, it would take many games to reach that point; not to mention that I never, ever spent 6 or up to 10 hours sessions playing either those games or even any other memorable and personal Hall of Fame games that I hold near and dear to my heart. There's indeed a clear distinction between loving a game but "just" playing it 30 or 40 hours, and dedicating yourself to a game and playing it for months almost without pause.