I think that depends. Many of my homeschooling friends work for as little as 2-3 hours a day for five days a week, and others work for 5-6 hours a day for three days a week, etc. Other homeschooling families make school a hands-on part of everyday life, so the students are "in school" almost 24/7. ;-) It really depends on how much work the parent sees fit to assign, what kind of work the parent assigns, and on how fast the child/children can work.
I am homeschooled and will be a Jr in high shool. I do my school pretty much in my spare time because I am so busy. I would say I work on my school on average 6 hours a day. (five days a week) What it really comes down to is if i get all my assigned work done, rather than how long i spend doing it. For example it might take me an hour to do my math one day, but the next day it takes me two hours. That is why the time varies so much. It just depends on the student.
A guideline is 4.5 hrs max. Miz
I homeschool my 7 kids for 7 hours. I guess with 7 kids you just have to get the job done and for me the only was to do that is to wxtend our school by an hour.