Here's a rough guesstimate. Disclaimer: it's very rough.
5 year old child (for argument's sake) in every 24 hours might spend:
11 hours sleeping
1 hour eating supper, having bath, getting undressed and ready for bed
45 minutes getting up, dressed, eating breakfast, getting school bag ready etc.
6.5 hours at school
1 hour travelling to or from school
15 minutes doing homework
which leaves an estimate of 3.5 hours free at home on a weekday.
This assumes that the parental work schedule fits around taking them to school at school starting time, and bringing them home as soon as the school day ends - no after-school clubs in this scenario.
This also assumes that the child engages in no activities outside school and never has a friend round for tea.
My timings haven't allowed for time while Mum Makes Supper (half an hour preparation time?) which means that the free playtime with the potential of playing with Mum is down to 3 hours a day.
And if this hypothetical 5 year old is a 12-hours-a-night sleeper, then we are down to 2 hours a day of free playtime with possible parental involvement.
No need to quibble with the figures, which of course are not accurate for any family, but it's an illustration of why some HEing families just can't understand how mainstream families have time to do the whole school lifestyle thing.