Advance Wars DS...9 months and 170 hours later, I'm still playing it.
I'm a completionist at heart, so I've definitely spent way too much time on games in the past. In recent years I've had less and less time to devote to dozens of hours to a single game, but that all changed with Advance Wars DS. I've never even played the first two (although my brother was a huge fan of them), but so far I've logged about 85 hours on it since I got it last month. The game is just so damn addictive, with way too much to do. I'm just getting started on the War Room maps, yet I already can't wait for a sequel.
Somehow, I ended spending ~130 hours on Samurai Warriors and SW: Xtreme Legends combined. I don't want to know how long I've been playing all of the Dynasty Warriors games combined.
3D platformers have sucked up a lot of my time, too. Donkey Kong 64 was an absolute nightmare to complete. I also spent quite a bit of time on the Ratchets and Slys. Jak II was just a nightmare in general.
I remember spending 120 hours on Final Fantasy X, 30 of which were devoted to Blitzball alone (and I hated Blitzball). I'd say another 30-40 hours were spent reconfiguring the Sphere Grid and maxing out stats as much as possible for 3 or so characters. FF X-2 was less addicitive, mostly because there was no way in hell I was going through Blitzball again, especially w/o being able to control the characters.
FFIX consumed my life just because of the Chocobo digging (still my most favorite Chocobo mini-game ever). FFVIII came pretty close just because of the card game.
Going back a bit more, FFVI definitely took up a lot of time. I didn't quite max everything out, but I remember coming pretty close. I remember trying to get the Paladin Shield, but lost track of how many battles I fought and so gave up on it. I'll make sure that changes for the GBA version
Writing FAQs pretty much necessitates playing through a game multiple times. So many times, in fact, that I'm now sick to death of the Zelda Oracle games, Sword of Mana, and pretty much any game I've ever FAQed. The only exceptions to this, however, are the Wario games. I must've played completely through Wario Land 3 at least 4 or 5 times, but I'm still not tired of it. And, of course, the Wario Ware games are so quick, fun, and random that I'll never ever tire of them.