It's likely that a class will be between 40-50mins, therefore you'll be teaching between 14-17 real/clock hours a week. Per week I'd guess you'll only actually need to plan 4-5 classes and then just repeat them to different sets of students. It's an easy, if not slightly boring job and 8000rmb a month is above average for doing it. You can earn more in other parts of the world teaching in different types of institutions but I'm sure with your experience you already knew that.