i rarely play for more than 3 hours anyway, and, even then, i'm right next to a power socket. I don't play for long periods of time in the car as i feel sick and i can't imagine the 3d will help. Actually i think probably most people will feel travel sick while playing on lowest brightness (having to strain eyes), with full 3d, on a games console for 3 hours while going round all those bends in the roads.
Also, another point, if nintendo did actually say this, then it's probably just so people can't sue, and realistically it's probably about 5 - 7 hours.