Background: Guided gaze is the instructor's gaze towards teaching materials to guide students' attention, and it plays a vital role in enhancing video-based education. The duration of guided gaze, indicating how long instructors focus on teaching materials, varies based on the lecture design. Nevertheless, the impact of varying durations of guided gaze, especially concerning students' prior knowledge, remains inadequately understood. Objectives: This study investigates the influence of the instructor's guided gaze duration and students' prior knowledge on learning performance and affective experiences in video lectures. Methods: 145 fifth-grade students participated and were divided into high and low prior knowledge groups based on a pre-test. Within each group, students were randomly assigned to view one of three video lectures with different guided gaze durations (high vs. medium vs. low). Learning performance and affective experiences (learning experience, satisfaction, and emotions) were measured as dependent variables. Results and Conclusion: The results revealed that low guided gaze duration significantly improves learning performance for students with low prior knowledge. Conversely, high guided gaze duration negatively impacts learning experience, satisfaction, and positive emotions. Additionally, students with high prior knowledge reported higher learning experience and satisfaction. These findings highlight the interaction between guided gaze duration and prior knowledge in students' learning performance. Implications: Our findings provide valuable implications for the design of guided gaze duration in video lectures based on students' prior knowledge. By adjusting guided gaze duration appropriately, instructors can optimise students' learning performance and affective experiences.