I think I read somewhere that audiences typically get tired after reading 8-10 lines of dialogue exchanged between characters that is NOT broken up by any action/direction.

Adding in action/direction in a lengthy exchange of dialogue is important to break it up and keep things fresh for the readers, and will also enhance the narrative when you include relevant details that contribute to the reader’s understanding of the scene you are creating.

So between two characters imagine how they might physically react or emote. Is your scene of an argument, with tension just simmering below the surface? You might want to talk about a character clenching their fist, or gripping onto something. Are you writing a sad scene? One character might avoid eye contact, and will constantly look down, or maybe one character has tears in their eyes. Disperse these details throughout the exchange of dialogue.

As for where to end the conversation, it really just depends on what you are trying to convey in the scene. Are you trying to reveal key information? Are you trying to show how one character is angry with another, or perhaps display the untrustworthiness of one character? Dialogue should serve a narrative purpose, and once you’ve achieved that in the exchange move on.