Well I always retained information quite well, so homework normally only took me 20 minutes to an hour to complete. But it is true, going to school longer just burns you out. Say you have an hour long lecture, I mean straight lecture(college I had 3 hour long lectures, ended up playing on my laptop after the first hour and still aced the tests)it is said that only the first 20 minutes of that will be absorbed, and the last ten minutes will be absorbed. That leaves half the time where nothing is being done.
Now if American schools were like academies where the students lived on campus, I can see having longer school days would work. You can have more and longer breaks between classes, giving the kids time to cool down after a brain workout.
The one thing that screws kids up these days is being helped too often I believe. There was always 1-5 kids near the back of the class who needed the teacher to do everything for them, this slowed the pace of the whole class down. It wasn't that they were slow or stupid, it's that society has taught them that they can always get someone else to do something for them, and that's why we do not cover as much material as we should.
I had a physics class once, where we were supposed to cover the whole book( 10 chapters) in 18 weeks. The first two weeks we covered a chapter a week, being just simple measurement review and such. But once we got through the common knowledge base(should be common by now, we were seniors) we ended up slowing waaay down for idiots who wanted the teacher to tell them how to do something every time. We ended up barely entering chapter six before it was time to graduate. I took the book, sat down and read the thing myself so I may at least learn something from the damn course.
So yes, how US schools are designed, longer days will be bad, the problem is not the time allowed, but how it's used. I blame it on society, which is getting lazier and lazier.