That's not necessarily true though. Some people can easily adapt to the use of 3D while other find it extreme difficult if not impossble to given any period of time.
I bought the 3DS at launch down here and have had it for over a year, in that time I have only been able to stand playing the thing with 3D on half way, and even then it is for about 10-15 minutes tops. After that period of time my eyes begin to get sore and my head begins to hurt.
The 3D effect effects all people differently, some people can stand it with ease, others will struggle with it regardless of how long they try, and other won't even be able to see the effect at all.