Where eyes don’t hurt.
Stereoscopic 3D zone of comfort is the range of positive and negative parallax (apparent depth behind and in front of the screen) in which discoupling of accommodation and vergence can be achieved without discomfort.
Excessive parallax between the left and the right eye view on the screen causes eyes to converge or diverge too much which can cause eye strain. The actual boundaries of zone of comfort vary slightly from person to person.