TWEWY was plenty long, that kept me going for ages. Pokemon Pearl/Diamond did as well, though not as much as previous Pokemon titles. Final Fantasy Tactics A2 has chewed up a lot of my time too, and I still have a lot to do in that (though I've had it sitting on my flashcart doing nothing for months now, I don't think I'll get round to it ever).
Wild World kept me entertained the whole of the first holiday I had with my DS (no flashcart ofc), so I musta spent a long time on that (amazing to think that was about 3 years ago, and the DS is still going strong).
EDIT:
Oh, almost forgot EBA and the two Osu! games. I still go back to them occasionally, and must have clocked up absolutely hours in the couple of years I've had them.