Elementary Grades you can estimate 1 hour per grade. So a third grade child can expect to take 3 hours per day of study.
The K12, public school at home program, can average around 6-8 hours per day.
Realistically, you can all expect to max out at 7 hours per day; because more is just too much to handle.
Children with Attention or Learning Differences may take as much as twice that, with close parental involvement.