Inspired by complex system theory, this study adopts a multilevel piecewise regression model to examine the relationship between homework time and the development of students in different grades using a large-scale testing database in China. The results indicate that the relationship between homework time and students' performance is positive when homework time is less than the cut-off values and becomes negative when homework time is greater than the cut-off values. For different dependent variables, the cut-off values of homework time differ but are all within a range (1-1.2 h for grade 4 and 1.7-2 h for grade 8). The reasons for the difference in cut-off values among students in different grades are discussed.