I was recently asked this question by a reporter doing a nationally syndicated story on childhood education standards and practices. Admittedly this is quite a broad question, with many considerations (age/grade of student, achievment level of student, type of school, etc.).
However, really homework should be based upon quality rather than quantity. Too many sub par teachers, following outdated curricula are in a way "outsourcing" the education process directly to the students, and ultimately to the parents as well, since parental involvement is a key factor in a students rate of success. A lot of teachers utilize their only actual face time with students (i.e. class room time) to broadly touch on subject matter, then assign prodigious amounts of home work that is in essence saying, "here is a little bit of information about a lot of things, now go and learn it yourself". This is not always the fault of the teacher. In many states, recently adopted common core curricula dictates the breadth of information to be covered, by subject and by grade, and this does not always allow for teachers to engage students individually, based upon their various needs. Teachers end up teaching to the standards, not to the class.
Ideally, homework should allow students to further explore specific subject matter and expand upon the foundations of in-class lessons and discussions. In addition, an effective homework load should teach them self reliance, organizational skills, critical thinking, preparedness, and the ability to recognize the importance of meeting deadlines. But in reality, too often homework results in the opposite: reliance on parents, siblings or tutors; disorganization and feeling overwhelmed; lack of focused thinking; unpreparedness and anxiety; missed deadlines and falling behind. Whether or not a student's end result of homework is the former or the latter depends heavily upon the passion, skill and effectiveness of the individual teacher that assigns it.
So it is difficult to put an amount of time as being just enough, or too much, when analyzing homework loads. If one goes by historic assumptions, then the rule of thumb is roughly 1/2 hour of homework per subject, per night. However that is not the way any modern educator would answer this question. Effective homework assignments should emphasize and reinforce the classroom teachings and discussions, and encourage (not discourage!) further exploration of the subject matter. When homework fails to do that, no matter how much or how little the amount, then it is too much.
At FutureSoBrite we believe that every child deserves a bright future, and that education makes all things possible. We believe great education doesn't just happen in school, it also happens at home. We are educators and parents who believe lessons learned at home are the foundations for lessons learned in life. Because teachers teach the class, but parents teach the child.