I'm sure I've put hundreds of hours into Super Mario Bros. 3 in its various incarnations (NES, as part of the All-Stars collection, and then on its GBA release). As far as RPGs go, the one with the longest playtime for me was easily Dragon Warrior VII, which I put 144 hours into, according to the game clock. Considering how many times I replayed Final Fantasy VI when it first came out (at least 3 full playthroughs, each one clocking in at at least 60 hours), that one qualifies as well.
What about you? How many hours of your life have you wasted on a single game?