There are no hourly minimums for a school day. The law requires that homeschool instruction be conducted 180 days out of the year, but what you do on those instructional days is up to you as far as what you think works best for your child. If your child is breezing through what you have planned, you may not need to spend as much time "doing school" those days. Other days may take longer, still other days you may decide that a field trip will be your school day. You can hold school during the day or the evening and work it around your work schedule.