The first game I remember playing that had a timer was SNES Final Fantasy 2. Our first run through took me and my buddy about 30 hours. Eventually I got so good at that game I could get up to the final boss with 8:30 on the timer. But that requires mastery of the game few possess or care to acquire. Playing FF2 over and over again is pretty much pointless. Mostly because its exactly the same each and every time. At least FF3 would be a little different depending on who you had in your party and what Espers you focused on.
I dont even wanna guess how much time I've put into Final Fantasy 1. By comparison to today's games, that sucker was a HUGE grindfest of leveling up and saving to purchase over priced goodies.
When I had the Playstation version of FF7 I put about 80 or so hours into it, mostly to level up and get a buttload of master materia so I could beat the Emerald and Ruby Weapons. Then I went and stomped Sephiroths butt in about 3 rounds. On later run-throughs of the Windows version, I didnt put in nearly as much time trying to master materia and such.
First run-through of Baldurs Gate 2 I'm sure I put in about 200 hours. Eventually I would learn the game very well and also use a character editor to increase my PC's run speeds. That doesnt do much of anything in combat, but it speeds up normal gameplay considerably. Same with the Turbo code in Knights of the Old Republic 1 and 2. Those games are painfully long seeing as most of the time is spent jogging around. With Turbo speed I can get a normal game done in about 12-15 hours. Without it my completion times are closer to 30 hours.
Morrowind and Oblivion its hard to say since I have been through several characters on each game. I'd say my longest for Morrowind (in one character) was about 600 hours. Longest for Oblivion was close to 1000. Yup, despite the fact I loved Morrowinds gameplay much more, I actually spent a little more time with ES 4, probably because of Mods. Also with both games I often get tired and find a super encumberance mod and also artificially increase my running speed.
Deus Ex I've done more than a dozen times. Wish I could tell you an average for each run through, but they get considerably faster as I learn it better & better. The running augmentation does not seem to appreciably affect the length of the game, since you have to turn it off constantly to save power. I'm never able to keep it active throughout a level.