How long is a ‘conversation’ in an open office/floorplan?

How long does etiquette dictate that it is a ‘meeting’ instead?

There are no offices or walls, even for the top managers in my building. Four clowns with exceptionally loud voices insist on holding conversations at full volume, all day long, about every triviality of their ‘work’ and personal lives. How long is it reasonable to consult, and after how long should they adjourn to a meeting room?

There is never any shortage of meeting rooms in this building, just inconsiderate, obnoxious unprofessionals.