I believe the truth is in the middle. If the school already does the bulk of the work, it seems like overkill to do too much additional work - there are diminishing returns. At state schools, however, there isn't much (if any) exam prep in class, so it makes sense to do more at home.
As with adults, there are good and bad days, but it is difficult to fathom how can consistent results be achieved with such long hours of work daily - if it is about clocking the time - it is possible to "do" two hours a day, but are the results consistently high, or is the practice productive? I am not arguing, just genuinely asking.