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

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.

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

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

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

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

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.

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.

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.)

Anywhere between 8am and 4pm different subjects can be scheduled, each being 50 minutes. Every day can be different, sometimes there's empty hours between classes. Some days you might have 2 classes, other days 6.

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.