In poke'mon Fire Red, it only took me about 300 hours to get every poke'mon and train them to level 100 each, half the time for Emerald seeing how I had all of them on Fire Red and you can easily just transfer from one game to another. Never got to finishing the other games, didn't have a cable link or spare gameboy for gameboy color games, and the rest of them bored me too much to want to actually get through it. I refused to get any further than the poke'mon league in Diamond/Pearl/Platinum because they sucked that bad. Can't wait for Soul Silver and Heart Gold versions though.
If you're talking about time for actually playing the game, despite beating it numerous times, Oddworld definately tops, seeing as I've played it non-stop since I was about 4ish or so.
Now, as for actually trying to beat a game and no matter how hard I tried I just couldn't, the first Rayman game took me YEARS to beat. No, really, we got it when it first came out and ever since I had been trying to beat that game like crazy, then I finally beat it and shit was I disappointed. Rayman games are great and all that, and really fun to play, but when you finish it it's pretty frusterating getting such a crappy ass ending.
The majority of other games I've played on average take me about 3 days tops to beat all the way through and 100% on everything.