I think if you stripped out everything that wasn't learning, school would probably take about 12 hours a week. I base this on my daughter's school, where she has 12 hours a week of instruction including: math, language arts, science, social studies, art, music, and technology. Schedule-wise, this is three days a week 9:30-3:30 (includes an hour lunch and 15-minute breaks between classes). There are usually a few hours of homework a week, which she has plenty of time to do (we do expect this to go up as class levels do). PE is covered by swim team and martial arts outside of school and she also has time to socialize, read for pleasure, pursue hobbies, and lay around lazily avoiding chores.
Because of the odd nature of the school, I can't say for sure that's she's learning everything she'd learn in regular school (but regular school wouldn't let a 10-year-old take middle school classes, so it's a moot point). I can say that in her opinion and mine, she's learning more than she ever did at public school and she's hardly in danger of falling behind.