The abstract of the paper is at
http://www.tandfonline.com/doi/abs/10.1080/00220973.2012.745469?journalCode=vjxe20#.Uz6wcPldXHV . It says that homework for high school students has diminishing returns beyond 2 hours a night.

http://www.oregonlive.com/kiddo/index.ssf/2013/09/back_to_school_homeworks_benef.html
Homework's benefits questioned in new study
by Amy Wang
The Oregonian
September 05, 2013

Quote
A study published this summer, "Nonacademic Effects of Homework in Privileged, High-Performing High Schools," asserts that current homework practices in such schools "sustain students' advantage in competitive climates yet hinder learning, full engagement, and well-being."

The study's three authors include Mollie Galloway, assistant professor of educational leadership at Lewis & Clark College's Graduate School of Education and Counseling, who says by email:

"As parents, we often make assumptions that homework is good for our kids. We assume it promotes learning, responsibility, study skills, and other positive behaviors. The research on homework is less rosy. There is no evidence that homework at the elementary level enhances student achievement, and at the middle and high school level, our study suggests that too much homework can, in fact, have detrimental physical, mental, and social impacts."

The study, which Galloway conducted with Jerusha Conner of Villanova University and Denise Pope of Stanford University, used data from online and paper surveys of 4,317 students at 10 high-performing high schools in communities with a median household income of more than $90,000 per year.

The students answered questions about homework load, homework usefulness, stress, physical health, time for other activities/endeavors, behavioral engagement (for example, how often they tried as hard as they could in school), and demographic and achievement information.
On average, students said they spent a little over three hours a night on homework, work that they generally found "somewhat" but not "very" useful for learning the material. Most of the students, 72 percent, said they were often or always stressed over schoolwork.

The researchers also found that the more homework students reported doing, the more they reported school stress, physical distress, inability to find time for friends and family, and likelihood of dropping one or more activities. Students also reported many "pointless, mundane" and time-consuming assignments.

When asked why they continued to do homework, many students said they were anxious about disappointing their parents and being punished, threatened or ignored if they did not produce good grades, often defined as straight A's.

The study also noted that:

The benefits of homework appear to plateau at about two hours a night for high school students and that beyond two hours, homework may have "detrimental achievement effects."
Students often find homework less engaging than other out-of-school activities, with the exception of homework that is "authentic" and centers on solving real-world problems.
Homework can diminish the quality of family interactions and foster conflict between school and home.