From the age of 7 or 8, they are doing full days at school and at least 1 after school hagwon (private academy). That ramps up as they get older. By 10 or 11, they are doing full 9am-9pm days. Middle and high school kids are more like 9am-9pm in class and I've had kids tell me that they go home and study until 2 or 3am, regularly. I routinely had kids breakdown and cry.
After a while I realized that in a small way I was helping contribute to the continuation of this lifestyle and I soured on the whole teaching English in Korea thing and moved back to the States. I loved Korea on the whole and would go back in a heartbeat, but not as a teacher.