Summary:
Obligatory: in front of non-Muslims, since they do not believe in lowering the gaze.
Optional: if covering the face among non-Muslims itself poses a hardship, then she chooses the lesser of the two evils.
Recommended: In front of Muslim men who are not lowering the gaze according to an opinion.
Permitted: in front of Muslim men who are not lowering their gaze according to one opinion.
Discouraged: when there is no reason, as this is excess in religion, even in front of Muslim men who are not lowering their gaze.
Discouraged: in salat if it wraps around the face since it is a barrier between the face and sujud, but if it drapes from the top, then it is not discouraged.
Forbidden: in the state of ihram, unless the cloth drapes from the top, in which case it is a head covering not a face covering.