I know Spanish often avoids the passive voice by using the active instead or 'se' to change the subject of the sentence, but when do Spanish speakers use the 'pure' passive with `fue/fueron [past-participle]? When it the passive voice appropriate to use in Spanish? How can I tell when a sentence would be best translated by using the passive voice? When do native speakers use it?
Generally, native speakers prefer the use of reflexive passive voice (voz pasiva refleja) over the normal passive voice. The reflexive passive voice, which is pretty common, is really a "disguise" for the normal passive voice. Its usage is quite similar to the active voice, but adds the pronoun
se to the verb.
Los delegados aceptaron la propuesta. (active voice)
La propuesta fue aceptada por los delegados. (passive voice)
La propuesta se aceptó. (reflexive passive voice)
As you can see in the examples above (which were taken from here), the three sentences have the same meaning, but the reflexive passive voice omits
los delegados, i.e., the agent of the verb (the entity that performs the action).
So, you can imagine that passive voice is used when the agent of the verb is important and must be present in the sentence, but not as important to be the subject in the sentence. The passive voice is more common in formal contexts, especially in written texts, but not in common speech.
You can also find a more detailed analysis of the reflexive passive voice here.
The most likely answer is never. In castillian, passive voice is rarely used except in articles for the media or some other not-casual language.