depending on your local office. Use BCIS case status online https://egov.immigration.gov/graphics/cris/jsps/index.jsp?textFlag=N# and
look up the processing time of your local office.
You don't need GC to go back to China. Just file for AP (advanced parole)
together with your GC application. You should get AP relatively soon. The
processing time of AP (I-131) is also in the website above.
Once you file for GC, you will not be able to get F1 visa due to immigration
intention.
Search the BCIS w