Using data from the China Education Panel Survey, this study examines the impact of homework time on academic performance among Chinese junior high school students. The findings show that seventh and ninth graders spend 2.28 and 2.67 hours daily on homework, respectively, exceeding the 90-minute limit stipulated by the “double reduction” policy. Homework time significantly varies by gender, grade, and region. The relationship between homework time and the academic performance of junior high school students follows an inverted U-shaped pattern. Ordinary Least Squares analysis indicates that the optimal homework time for maximizing academic performance is less than 0.96 hours per day. Beyond the threshold, the positive effect diminishes, and when homework time exceeds 4.07 hours, it negatively impacts academic performance. Mediating effect analysis shows that excessive homework time leads to negative emotions, such as boredom, unhappiness, frustration, and sadness while depriving students of necessary sleep for healthy growth. It is concluded that optimal daily homework time is about 1 hour, aligning with the “double reduction” policy guidelines.