I have a 1800 with outbound QoS enabled and outbound speed set to 3900 kbps, it runs in oversubscription mode, my question is does the box do shaping to 3900 kbps so you can se outgoing traffic clocked out with a speed of 3900k?
Most of the WXC feature details are covered in the WXOS Admin Guide and the Technical documentations presented in the Juniper site.
Those documents were prepared for the public/customer reference. Mostly you should find an answer for your queries over there.
Regarding your question , "how long time/many bytes do the box serve before going to next traffic class" , i dont think there is a straightway answer for this in any of the document available to public.
May be you can get in touch with your Local Sales Engineer / Technical Marketing Engineer who can provide more insight on this.