Remember that comparing hours is a relative exercise. Some kids work faster than others, even if both are paying full attention. So, as usual, teach the child, not the curriculum/schedule. 
That being said, Charlotte’s 7-9th graders did their work in 4 hours (9:00am – 1:00pm). Afternoons were for nature study, handicrafts, etc.
One other thing to keep in mind when talking hours: if you have an opportunity to visit a regular classroom-type school for a day, you will see that though the kids attend school for about 6 hours (if you subtract lunch break), they are “on task” for only a fraction of that time. The rest is spent in classroom management, distributing materials, getting everybody on the same page, answering questions (actually, repeating information that the student missed hearing somehow), etc. So don’t panic if your children can get their work done in a shorter time frame than their classroom counterparts. The main thing is that they’re progressing and learning.
