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

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.

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.

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

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.)

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.