I would estimate, given what i have seen in regards to homeschooling and the records of prestigious schools that in an eight hour day there is only really about 4-5 hours of actual learning/absorption of knowledge/synthesis/integration at best, likely significantly less
And yes, burn out is definitely an issue as well.
Very few children really thrive in the way schools are structured now, those that do are rare or given outside help & instruction that a lot of children just can’t access (that whole classism thing again)
Like, I was a fairly intelligent child, but also had undiagnosed/untreated adhd/mild autism, i was able to actually progress as well as i did because of my intelligence & a fortunate overlap in special interests.
I didn’t really start actually learning until college when i was more able to self direct and seek out instruction methods more suited to my ways of learning
I mean i didn’t say i learned a lot from my professors
(Although i did, as they had more freedom to teach and track student interests into account)
But just not being restricted to what i was “supposed” to learn or one particular methodology helped more than anything.
Being allowed to follow an interesting topic brought up in class and study it and write about it with the approval of the teacher even if it wasn’t strictly on the syllabus was amazing.