It sounds like someone wants you to fulfill some "40-hour" work week.
My university "requires" office hours, but doesn't put any stipulation on them. I teach 14 hours a week and I have office hours (or "free talk" as I call them so the students understand what happens there) 2 - 4 hours a week. I find out the students' availability and try to schedule free talk during that time.
Sitting in your office for 24 hours? No, not common! Is the administration wanting you to edit papers, etc., during that time? Different skills for that job! You'll need to point-blank ask what are their expectations for office hours. If it is so students get private tutoring, then you should point out that that takes extra preparation, etc. Additionally, if they want you to do a quality job on the 16 hours of other classes, then you need time to prepare for those as well.
It takes me between 4 and 8 hours to prepare for each "different" 2-hour class. I know there are many foreign teachers who just "get up and talk," but I'm not one of them. I currently have two different class preps a week, so if I had to sit in an office for 24 hours of the week, I'd be screwed, unless I was able to use that time to prepare.
Additional time is needed for any evaluations you may give and for preparing feedback for the students. (They can't improve if they don't know what areas need improvement.)
5000 yuan seems reasonable, depending upon your area. I know it's more than the teachers get paid out here in the Northwest.