In general, I never really have my 3D on. It always distracts me and it makes the battery drain faster than you know. I sometimes turn it on at credits or key scenes in games though but most people I know have there's off completely or on a tiny bit.
Although I don't have a Japanese 3DS so I don't have New Leaf yet, I always use 3D for all my 3DS games. I can go with it on for hours and hours and not have any eye strain or headaches. I plan to use it all the time on New Leaf as well when it reaches the USA.
I don't play the Japanese version, but for any other 3D game, it's off. AC:NL, it will only be turned on in a rare occasion, and at a very low setting. I get a headache from that small of 3D viewing. ><
I never ever turn 3D off. When I do it is to see the difference and I really enjoy to have depth.
For those really curious and that didn't know, you can find some 3D screenshots on the internet and take a look at them on your 3DS, like this : http://www.gonintendo.com/?mode=viewstory&id=190000
I think I realized why I can't look at 3D for too long. It's probably got to do with the fact that my eyes aren't perfect. Even though I just need glasses to see things far away, probably makes focusing on things like 3D painful for my eyes.
I regularly use it, actually. Sometimes I would have it on full, but that would only cause me to see double image. Most of the time, I have the setting on halfway, because I like the 3D, and at the same time, I wouldn't experience the harsh double image.
