I would say an era in boxing is a time span in which a division is suddenly filled with promising and highly skilled boxers that face off in an elimination contest. When this happens the division is stacked. When it doesn't happen we remember back at the "golden eras" and hope for another one. Often one fighter can rule a division but its not necessarily a great "era" unless the comp is there, just a great fighter. It's very subjective but you know an era is an era when its over. Just like great fighters they get their full worth when they're no longer there.
Yes you are right. An era doesn't have to be long. A boxer/s that defines an era doesn't stay prime for too long simply because boxing is a brutal sport. An era could be as short as say 4 years. The Klitz doesn't have crossover,mainstream following. The US basically have a lot of say on who defines an era, which is why boxers who want to advance their careers go there.
i dont think p4p is as bad as era, personally i never use era unless im talking about my own era, i might say floyd is the best fighter in my era meaning my lifetime, for fighters era's i dont have a clue what it could mean