Games that I have spent an incredible amount of time playing, in no particular order (games where I have noted time, I have played those exclusively without playing any other games):

• Enter the Gungeon (two years)
• Master of Orion (& sequels and likes)
• UFO/TFTD
• Sim City 2000
• Command & Conquer (just the first one)
• Quake World (5 years)
• Ragnarok Online (5 years)
• Transport Tycoon Deluxe (& Open TTD)
• Wizardry 7
• World of Warcraft (5 years)

I'd like to mention that there comes a certain point in skill based games like fps and twitchy MMOs, where your character's movements are so ingrained in muscle memory, your knowledge of the game engines' ins and outs are so thorough, that even just standing around in one spot you know at any moment what action or reaction is going to happen upon your next few key presses, and you have a hundred percent confidence in executing those without a flaw, if you bothered to lift your finger.

That's when you quit, I guess.

Edit: honorary mention for Fallout 1/2/3/NV, TES3&5, Elite/First Encounter, Syndicate, Starcraft 2, Creatures 1&2, Civilization, Master of Magic, King's Bounty & HOMM, DOOM 1/2, Scorched Earth/Worms, Diablo 1&2