What is "normal viewing distance"?

When people talk about large size prints they also mention when the print is viewed at "normal viewing distance". Is there some general guideline? 3' for 16x24? 4' for 24x36? Less? More?

The rule I know, others may have different rules, is that viewing distance equals the diagonal of the print. So for 16x24 about 29 inches, call it 2.5 feet. And for 24x36, 3.5 feet.

Of course a lot depends on the sort of friends you have. If they are photographers, expect to find their nose prints on the glass.

It also depends on the image IMO: A pano shot of a landscape, with lots of fine detail is more likely to be viewed from up close then a headshot.

The definition even varies...one rule is "diagonal dimension of the print", another rule is "1.5 * diagonal dimension of the print", it just depends who you are talking to.