Processing times vary between the centers, but it could take up to 90 days. Keep in mind that your petition for renewal will allow you to continue working while you wait for a response, so long as no denial was issued.
I would file a petition for renewal of you work authorization. This should cover you will the renewal is pending. The processing time should be less than 90 days. However, there are no reliable processing times with USCIS. Try this link:https://egov.uscis.gov/cris/processTimesDisplayInit.do