Final answer:
For a long presentation lasting all afternoon, it is best to give a coffee or stretch break every 120-minute period to maintain audience engagement and concentration.
Step-by-step explanation:
The question refers to the optimal timing for breaks during a long presentation. Based on the provided information, it is recommended to schedule brief breaks for increased productivity and effectiveness. Specifically, taking a ten-minute break every two hours is suggested to enhance overall accomplishment during activities such as studying, attending long presentations, or performing any extended tasks.
Applying this to a long presentation scenario, if a presentation lasts all afternoon, including a coffee or stretch break after every 120-minute period would align with the recommendation. This practice would help to maintain audience engagement and optimize concentration levels.