1.Kid Icarus: Uprising 143:55
2.The Legend of Zelda Ocarina of Time 3D 140:27
3.Web Browser 78:42 (I read manga on my 3ds)
4.Mario Kart 7 37:40
5.Freakyforms 32:05
6.Super Mario 3D Land 24:13
7.Fire Emblem: The Sacred Stones 22:27
8.Tales of The Abyss 21:09 (Still playing)
9.Kirby and The Amazing Mirror 20:47
10:Pushmo 18:20
11.Zelda: Four Sword 17:58
12.AR Games 16:04 (I must have left it open, because I didn't play this so much)
13.The Legend of Zelda: Link's Awakening DX

The rest are useless things, demos and Dillon's Rolling Western that I didn't play yet

The single player campaign lasts around 10 hours, but there's lot of replay and the online, great game, you should buy it.

This seems like a fun thread. Unfortunately, most of the games in my Activity Log are DS games. I never had a DS, and they're way cheaper than 3DS games! Actually, Kid Icarus Uprising is still my only retail 3DS game. I've had a blast with eShop & VC titles though.

I always check how long I've played certain games for, and mine is kind of insane. Brace yourselves.
Top 20:
Kid Icarus: Uprising - 106:42
Ocarina of Time 3D - 93:57
Kingdom Hearts 3D - 93:16
Resident Evil: Revelations - 75:05 (I replayed this 3 times and played the online mode about just as long)
Tales of the Abyss - 73:53
Netflix - 65:52 (Summer break Netflix run ftw)
Theatrhythm Final Fantasy - 61:27
Internet Browser - 54:05
Mario Kart 7 - 47:42
Zelda: Link's DX - 33:13 (this was my first game, I played the game through 5 times)
SMT: Devil Survivor Overclocked - 32:31
Radiant Historia - 32:28
Nintendo eShop - 30:27
Final Fantasy IV - 29:44
Pokemon White - 26:38
(a Demo, not going to say which) - 20:00
Dead or Alive: Dimensions - 19:08
Colors! 3D - 15:31
Tetris Party Live - 14:07
Heroes of Ruin - 13:53

Top 50 all have at least an hour of play on it.

edit: Also, I have had my 3DS for just a little over a year now.

I'm curious, I'll guess it is Nintendogs

No, but I wont give any hints because there aren't that many demos to begin with.

I bet it's Order Up.

It's like you're begging me to tell you what it is.

1) Kid Icarus: Uprising -- 41:11
2) Advance Wars: Days of Ruin -- 23:49
3) Pushmo -- 10:36
4) Nintendo eShop -- 9:32
5) Star Fox Command -- 8:06
6) Internet Browser -- 6:35
7) StreetPass Mii Plaza -- 6:11
8) Mega Man Star Force Pegasus -- 6:05
9) Mega Man Zero Collection -- 4:24
10) Sakura Samurai: Art of the Sword -- 4:19
11) Mighty Switch Force! -- 3:12
12) Super Mario Land 2: 6 Golden Coins -- 2:43
13) Swapnote -- 2:00
14) Trauma Center: Under the Knife -- 1:58
15) Activity Log -- 1:22 (did this just get meta?)
16) Pokédex 3D -- 1:19
17) Nintendo Video -- 1:02
18) Notifications -- 0:59
19) Mutant Mudds Demo -- 0:58
20) The "Denpa" Men Demo -- 0:52
21) System Settings -- 0:39
22) Mii Maker -- 0:38
23) Super Mario Land -- 0:36
24) Metroid II: Return of Samus -- 0:33
25) Super Mario Bros. -- 0:29

after that it's mostly demos. For the record, I bought my 3DS at the end of this past June.

It is Farming Simulator 3D isn't it?