It depends on the office you are going to. If you don't meet the needs of that office (traveling within two weeks, don't have proof required, etc) the appointment may only be 2 minutes, with them explaining to you too bad, sorry, go away. Needing a visa is a valid reason for an appointment, but they may require proof of the trip to china to verify that.
If you meet all the requirements for in office service at that office, it shouldn't be that bad. They usually only issue numbers in fifteen minute segments, and at 9:30 in the morning they wouldn't have had a chance for it to start backing up all that bad yet.
Just make sure you have everything you need filled out and ready, I would guess probably a 15 to 30 minute wait at most, and 5 or 10 minutes as they go through your stuff.