when to look at camera and when to look away from, when being photographed?

When the subject looks at the camera, it invokes a different emotional response from the viewer than when looking away; it's the same feeling when an actor breaks the 4th wall in a movie.   Typically, when the subject looks at the camera, it creates an emotional connection in the viewer with the person in the photo or movie scene.

Not looking at the camera can produce a feeling in the viewer's mind that the subject in the photo is in deep thought.  This is something that cannot be achieved if the subject is looking at the camera.

When? When in doubt, do both. Decide later which looks better.