Home educators often try to replicate school at home when they begin homeschooling. At school there are lots of students and not a lot of one-one contact time between teacher and student. As homeschoolers we have the advantage of one on one tuition all the time so naturally our children will need less time per day.
Instead of giving them workbooks or busy work let them play and discover and investigate. Even baking cupcakes can become a science experiment by dividing the batter into three, add more or less baking powder and then see what happens to each set of cupcakes.
This is lesson time - breaks and outdoor activities or extra curricular aren't included.
You might be surprised but Homeschooling takes a lot less time than going to school all day.
20 mins of focused learning for a pre-schooler may not seem enough but remember they will be learning through play most of the time.
There is overlap in the older years because some students need more or less time than others to learn. Once a student is around 11 years old, they should add some extra private study time - starting from 1 hour in Year 7 going up to 3 or 4 hours in Y12 and 13.