How normal is fighting in relationships and what kind of fighting (duration, volume, weaponry.. etc..) is unhealthy for a couple?

For example.. If your neighbors can hear it, it must be unacceptable for a couple to continue, right?

What about if your roommates can hear it? Is it cool if it only happens once a month but uncool if it happens weekly? Is it uncool if it lasts more than 5 minutes at elevated volumes?

What are some good guidelines and what should observers and fighting couples do if there’s just too much of this stuff going on for anyone’s comfort.