how many days before USCIS will process H4 application withdrawal

I filed for H4 application in October 2017. However I found a job and the employer filed for H1B. So I want to withdraw my H4 application. As guided by my attorney I sent a withdrawal letter to USCIS requesting them to cancel my H4 petition. Do you know how many days USCIS typically takes before they can stop processing my H4. My fear is if my H4 gets approved first, I cannot work until the newly initiated H1 is approved. So I want the H4 to be withdrawn ASAP. Please let me know if I can expedite the H4 withdrawal process. I called USCIS and they say it can take up to 30 days to view the withdrawal letter and act upon it.