Animal Crossing is best played in short bursts. If you're playing for an hour or so every day, it's perfect, it'll last you for years, just make sure you have the sort of mind that can create its own objectives or you might run out of official 'to do list' stuff. Personally I sink a lot of time into redecorating my house. I WILL make it into a Doom Fortress.
If you need a real time hole, Monster Hunter is legendary for its ability to eat time, Tales of the Abyss is a decent 40+hour RPG, and there's always the mainstays of replayability like your racers, sports games, etc. I've also had to invest significant time into Donkey Kong Country Returns as it is kicking my arse nine ways to Sunday. I've been a gamer for more than 20 years and I STILL find this game pretty damn tough. If you want to get every Kong letter, every puzzle piece and every gold time trial medal, it should keep you amused for a fair while. Just be sure to invest in padded wallpaper if you buy because you WILL hurl your 3DS across the room.
