There is no single answer to the question of how much homework improves grades, as the effectiveness of homework can vary depending on a variety of factors, such as the individual student, the subject, the type of homework, and the amount of time spent on it. However, a number of studies have found that homework can have a positive impact on grades, particularly for students in elementary and middle school. For example, a study by the National Education Association found that students who did homework regularly scored higher on standardized tests than students who did not do homework. Additionally, a study by the University of California, Los Angeles found that students who spent more time on homework scored higher on exams than students who spent less time on homework.
However, it is important to note that homework can also have negative effects on students, such as increased stress and decreased motivation. Therefore, it is important for teachers to find an appropriate balance of homework that is both challenging and manageable for students.