I'd say that it would depend on the intent of the viewer and artist. If one is interested in the work for it's own sake, a reasonable distance would be more towards being able to view it as a whole and appreciate, composition, subject, etc. And this would change depending on the size. Don't think many artists produced their work to be scrutinized with a magnifying glass, tho some do. Stick your nose up to a Monet, probably not as satisfying as at 5 or 6 feet(further for his big ones). Saw an exhibition by an artist that did huge(12' X 16') photo realist paintings. When you walked in, it just looked like round blobs, step back 20' and it was a beach tide line, shells, glass, and all. In the same building were ink drawing that looked like large objects, trees, cliffs, and such. Walk up to it and it was all composed of minute faces and figures.
Each work is different and each viewer is different. Myself, while I have been known to nose press, I tend to concentrate more on the work as an artistic expression.
The first rule of art is that there are no rules...