a lot, usually several hours per week each for math, german and english, and some stuff for other subjects, but not regularly. I remember sometimes spending 3-4 hours a day doing homework and/or learning for exams in highschool, but a lot of people just copied the homework of others and most teachers didn't care too much (especially maths)

From grade 9-13, I went to a HTL (think of highschool with very strong focus on IT). There, teachers and students didn't care that much about homework, but we had to do A LOT of projects with very specific deadlines. thats where I found out that I can work on stuff for about 35 hours without any sleep if I'm really commited to the project (like start programming 48h before the final deadline and knowing if it doesn't work you will fail the class).