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My highest is Dota by a country mile (that's recorded anyway) prob around 1,000 hours, only like @Aroogah my time spent on D2 as a kid would have been very high. Also like @Sumo I've clocked endless hours on CS pre steam days.
Also this doesn't include any time spent on XBOX, just checked rockstar social and I'm around 300 hours on GTA online. That's probably my highest played xbox game.
What's the most time people have logged into games? Just a quick run through on my Steam library shows most games between 5 mins and 5 hours with a dozen or so games at 40-90 hours (Far Cry/Bioshock/Fallout etc). A handful of games at 100-175 hours (MGSV/GTA IV/GTA V/Kingdom Come: Deliverance/Stardew Valley) Then a few outliers:
Witcher 3: 210 hours
Skyrim: 240 hours
Mount & Blade: 350 hours
PUBG: 1110 hours
I guess that's just Steam so there's also Origin/UbiPlayer (BF3/4/1 and The Division probably has 200+ each)
I'd imagine that's probably over year of my life spent playing games (especially factoring in the days before Steam - I had tons of time logged in LOTRO for example). I do enjoy playing but that's quite a figure.