In World of Warcraft, type /played in the chat window and it tells you how long you've been playing total, and how long you've been playing at your current level. I'm surrounded by WoW junkies.
It's difficult to figure out which is longest for me. I've flown around the world in MS Flight Sim in a Beechcraft King Air... it took a very, very, very long time, lol. I remember it was about 19,000 nautical miles, I'd hazard a guess at maybe 150-300 hours total. I kept a log during the flight but can't remember off hand how long exactly it ended up being. I'm sure for total time played though MSFS has to take the cake, I can't even begin to guess how many hours I've logged in that game.
Otherwise, it's probably one of the GTA's, I've beaten all of them up to 100% except GTA4, I'm stuck at 92% and can't work out what I'm missing. I never use cheats or walkthroughs, I'm a purist gamer lol, except in games that never end like The Sims or Tycoon games, but even then I only use them sparingly

. Anyway, GTA4 probably takes the cake because I've spent an unholy amount of time trying to figure out what's keeping me from 100%.