When do you guys feel like you really need to turn the 3D off for a while? Could you play for hours with something like 30-40% 3D effect on? What's your experience with the effect?

Some games makes my eyes hurt. Others, like Mario Kart 7, does it perfectly. But I mostly just turn the 3d barely on.

I need to stop every couple of hours.

I don't get dizzy, but it's definitely a strain on my eyes; I think the longest I've kept the 3D on for is maybe 2 minutes or so... =/

Playing Ocarina made me want too puke. The only games I've actually enjoyed the 3D are Pushmo and Super Mario 3D Land. I've also learned that the handheld has to be a little farther, I was so used to my DS being close that initially I had my 3DS close but all that did was make me dizzy with the 3D on.

Yeah I can't play it for long.

It's no big deal really though. Once the games start coming out at a decent rate I wont ever be without my 3ds.

no

My eyes seem to get sore after a while but I finished mario and I started playing Zelda and not once have the headache or dizzyness kicked in. Keeping the slider at 20-25% seems to work well.

After a few hours yes, Nintendo does advise you to take breaks, now it is forcing your brain to ;)

I've had no problems with it except maybe when I'm on playing the train. I usually turn it off then.

No problems here.

I get da wizzy in da fizzy HAHA.

The first time i used the 3ds with the 3d on my eyes felt like they hurt a little, but after that day and using the best amount of 3d for my eyes i haven't felt a thing since that first day. Couldn't handle full 3D just makes me go cross eyed.

Yes at first week, but after that my eyes get used with the 3D , but i must take rest after 1 hour  of playing :)

I have yet to have an issue, I have yet to get eye strain either, and I play with 3D at max and I tend to play till the 3ds' battery is low.