By in game hours, I have no clue. But from the time I started playing it after I bought it, it would be the following games.
Zelda 1 & 2 - 5 or 6 years. If these games weren't so cryptic and difficult, they would've been completed much sooner.
Yu-Gi-Oh! Forbidden Memories - 7 or 8 years. Only managed to beat it once, and that was with the help of my brother's Meteor Black Dragon he gave me. Without it, I wouldn't be able to beat this game. I even got back to the final boss after playing through the entire game again while keeping my old deck, I couldn't beat him (and the other bosses with no check points in between at the end of the game) again.
PokÃ©mon Stadium - I still need to beat Round 2, and I've been playing since 2002 (or maybe 2003). If you consider completing Round 1 beating the game since Round 2 is pretty much hard mode, than it took me 3 to 5 years to beat it.