A lot, and that's why there's a problem, imo, not because of money.
When my kids were in public school, and spent a lot of time volunteering in the classroom, and I'd say that the vast majority of the day is spent doing other stuff: PE, eating, recess, getting organized, going to and from places. That's just the nature of a classroom full of kids.
Also they don't spend enough time doing the basics like math drills and the repetitive stuff other nations do. They tend to go at the pace of the slowest kid in the class. My second grader was still doing doubles addition at the end of second grade. This for a kid who already understood probability at the beginning of first grade ( because he watched Bill Nye the Science Guy every day ).
One day, near the end of second grade, I walked in and he was going over doubles addition with his seatmate . He looked at me with his eyes full of desperation and he said, "Mom, this is all the math we ever do ! " I can't stand it !".
That was the day we decided to homeschool. He graduates in May, and will start college classes this Spring.
Quite a few detrimental changes to our schools have been made since the sixties, curriculum and other aspects, and the results show it. But the only solutions the leftist controlled public education system will likely be just more money to do more of the same. And I'm sure home schooling and private schools will continue to grow.