Before I did homeschooling, I attended both private and public school. The private school had shorter days, less days all together, and probably a slightly lower homework load. All in all, I learned far more at private school then I did a public school. The nature of the assignments is far more significant then the time.
For example, in eight grade science class, we had no homework for an entire semester. The trade-off was that we had to do a large project, including summary of topic, background research, summation of findings, experiment proposal, final experiment plan, execute the experiment, collect data, summarize findings, draw conclusions, and present a final paper including a critique of our own possible methodological shortcomings and comparisons to conclusions of other experiments.
Contrast this with 9th grade (public school) AP bio class, where we had to read a chapter every two days and were graded on tests of what we had memorized. While I certainly learned alot, since it was learning in the "cramming" style, I would say my retention rate was only around 50 percent. I winded up getting an A in the class, but the lesson most of the students learned was to cheat. Rather then actually read the chapters, they would work in teams, each person coming up with definitions for a few pages of terms and short summary of important concepts and sharing them with their teammates. I would work by myself, but I eventually found to keep up, I had to just review the terms and concepts, memorize, and not actually read the chapter.
As time went on, the teacher found about the team method, and actually said this was the way to do it. The whole thing winded up falling apart when we realized he didn't even bother grading the tests and just gave generic grades based on how smart he thought individual kids were (really, we did an experiment, and realized he would only actually look at the first few and last few questions.) I fondly recall a friend of mine who had answers marked correct such as "Mr. ____ will not read this", "this class is a joke", and MOO!
He was eventually fired for sexual harassment anyway. But in all honesty, he was one of the better teachers I had that year.