USA: Typically for us, about 7-ish hours, I have to go in the building before classes start before then though, making it about 8 hours. Sometimes, we have half days, but they're pretty rare. We have all our classes, they're just very condensed 4-5 hours.

Here, we start at like 07:00 (Mondays 08:10 or so) and we end at 14:50.

Norway: 6-7 hours apparently, from 08:00-14:00 (2: p.m.) or 15:00. (3 p.m.)

The school I work for runs eight 7 1/2 hours but we could run 7 flat if we had a bigger building or less students. We need three lunch periods to fit them all in the cafeterias.

Italy: 5 to 6 h from age 6vro 19. For age 6-11 it's common to have longer school days (8:00-16:00) to give parents opportunity to pick kids after work.

For age 11-13 it's almost always from 8am to 13:00.

For age 14-19 depends on school, usually 30 to 36h per week, often on 5 days.

Rural France : 8 to 6, and half-days on Wednesdays.
Or rather, 7.55 (first bell) to 6.
Same for the middle school. My 7th grader's week goes:
Monday 8-6 (with a free half-period before lunch, and the regular 2-hour lunch break),
Tu 8-6
W 9-12
Th 8-5
F 10-5.
(But she's an overachiever and signed up for several electives. She could shave off 5 hours from her school week.)

We don't have high school in Ireland.¹

When I was in secondary school in the early 2000s, the day ran from 09:00 to 16:00 Mon to The, and we finished an hour early on Fri. My sister's school ran from 09:00 to 15:45 every day.

¹I am not being pedantic. Ireland, like Britain, has a two-stage system with primary (5 - 12/13) and secondary (12/12 - 18) schools, in contrast to the tgree-stage system found in France, America, and Japan.

Australia: 0900-1500. 6 sessions broken up by recess and lunch.

As an USAian, I was surprised to see 10 hours, but we don't do a 2 hour lunch here so it ends up about the same.

In China it's about ten hours, from 7 a.m. to about 5 p.m. It's going to last till bedtime if you're in the last year and be preparing for the college entrance exam.

Mine was 8:00 - 15:00 but I was pretty chronically late and tired because I couldn't fall asleep until around 2am most nights

it was 6.5 hours back in 1994 in the american southwest.

That sounds horrible.

Ah yes, the famous 996 culture.

In practice, is it 4 hours of paying attention and 6 of going through the motions, or just goofing off? There's been some research to the effect 8 hours already gives diminishing returns, if you're doing something subtle or complicated.

Thank you for the footnote. Without that, it could also have been pedantry, or the German system, where secondary schools have postsecondary-like specialisations already.

6-7 hours in India (0900-1500).

~6 hrs for me.
From 6 am to 11:30am

It could be shorter, but not by much: the school restaurant is a 10-minute walk from the school, and of course being French the food is ultra-local, organic, in-season home-cooked three-course meals (plus the cheese course, obviously), which you don't scarf down).
Still, it's annoying. My kid is very young still and 6 is very late for her.

I'm not sure how diminishing it would be, but as a person who went through it, most of the time I didn't nod off in the night-studying sessions. I actually had homework to complete and I remembered knowledge studied during the time. Even if it's diminishing it's still a net gain than those who don't do the extra study, given that I don't stress myself out.