If you end up digging them, DenpaMen will get you the most playtime for sure. There's a demo for it, by the way. It's full of charm, but to me, just a bland game underneath so don't leave it up to me to do the convincing.
Liberation Maiden is a great action game. Controls with one hand on the left side of the 3DS, one hand manning the stylus. For some people, this control scheme could be a deal breaker, though. Definitely short, but if you're into it, it's has a hard mode worth playing and some other "achievements" to expand your play time a bit. 3 to 4 hours all said. I'm very partial to it (thus why I recommended it), and I know a guy who's even going so far as to call it his likely game of the year.
NightSky is something else. A physics based puzzle-platformer, really it's more relaxed than anything you'll find on the eShop. I've seen some people/reviews call this one short as well, but playing the harder alternate mode (new levels), I'm going to get 5 hours out of this one, easy. Maybe more if the difficulty keeps going up. Your mileage may vary, I guess, if you happen to be particularly good at manipulating the little ball. It's got interesting, ambient music, if you were wondering about that. Highly recommended as well.
If I never reward you with attention, I may see you as a troll. Buy these: Liberation Maiden, Ace Mathician, Xenoblade, Solatarobo (sequel incoming, get hype)