"The amount of time students spend doing homework is related to their individual performance in PISA and to their school's PISA performance: students who spent more time doing homework tend to score higher in PISA, as do their schools."
"The net pay-off in mathematics performance from attending a school where more homework is assigned to students is particularly large - 17 score points or more per extra hour of homework -in Hong Kong-China, Japan, Macao-China and Singapore."
However " Pisa also finds that the average number of hours that students spend on homework or other study set by teachers tends to be unrelated to the school system's overall performance. This implies that other factors, such as the quality of instruction and how schools are organised have a greater impact on the school system's overall performance."
In my opinion, homework is important and necessary especially in subjects like mathematics. To understand the basic concepts and rules of mathematics is very important in the process of memorization and remembering. However, just memorization is not enough, not effective (and almost impossible) especially in higher mathematics. Up until high school, memorizing the rules without the deepen understanding can be enough. Students who try to "learn" this way, usually face growing problems during their high school years, and can be lost in calculus, statistics or other advanced math course. [not recommended]
15-20 minutes of practice, five times a week is enough practice if there is understanding behind it. Hours and hours of practice every day can still not be sufficient withouth comprehension. This is where a qualified, good teacher/tutor can help in a one-on-one set up. A good tutor can explain the small nuances that puts together the math like puzzle in a student's head. Piece by piece, all is going to fit together !
But likewise, just a good tutor isn't enough. (It is just like practicing withouth a good teacher, withouth understanding) It helps, but not too effective. Parents and students still have to commit their own share for the improvement. Commitment, time, effort, tenacity are all required to reach success!
The best teacher will fail if the student doesn't practice after she/he understands a topic. Because the information won't/cannot be transformed from the short term memory to the long term memory. It is simple. Transforming information from STM to LTM is more effective when students practice every day a little. Not once a week 3-4 hours, but every day 15-20 minutes. In this way math won't become boring and studying will become more effective.
In few words. Students need a good teacher/tutor, effort, and support from the parents.