I can't believe there is a need for such a story, but true—if you really think about it, the guidelines are much vaguer than one would really like. Here, the do's and don'ts of wearing dark lenses suspended in front of your eyeballs while in a human dwelling, as illustrated by 40 celebrities.
Okay: while performing "Tous les garçons et les filles" or any other yé-yé standard.
Not okay: while shopping. Too trying-to-obscure-my-identity-on-the-CCTV.
Okay: If you are very cool or established enough that you can do whatever TF you want.
Not-as-okay: If the sunglasses are more like saucers blown in from the early aughts. Also, if the wearer herself is from the early aughts.
Okay: If you are sitting front-row at a fashion show, because it does get so bright that it can be a bit difficult to see the clothes.
Okay: If you are arriving at somewhere (usually an airport) and you are either a celebrity attempting to avoid blindness or a civilian with gnarly under-eye circles.