Currently (as of February 2017) the USCIS processing times for I-751’s is 11 months and sometimes more. However, that typically fluctuates and can cause your application to be pending for over one year. You will not fall out of status even if USCIS is continuing to process your case past the one year extension. But to provide proof that your application is still pending and you are not out of status, you can attend an InfoPass appointment at your local USCIS office, where the USCIS officer can stamp your passport to show you are still in status. If your application falls outside the normal processing time, you can also call the USCIS main phone number and submit a case inquiry for them to check on the status of your case or place a service request.