USCIS will send you an acknowledgement letter within 20 business days from the receiving date of your duly submitted Form G-639, Freedom of Information Act/Privacy Act Request. This letter will contain a control number which is required to check your case status online on Official Website of the Department of Homeland Security (egov.uscis.gov/foiawebstatus).
So checking status of your FOIA request is easy, simply enter the control number into the provided field on the website. The status information there is updated every day. If your request is still pending, you will see the position of your request relative to all other pending USCIS requests in the same processing track. If your request has been processed and responded to within the past 6 months, you will see the date when your request was processed.
The processing time of a FOIA request can vary significantly, from approximately 4 weeks to 6 months, depending on the complexity, the nature, of your request. For example, processing of a request for entire immigration file will take much longer than a simple short request for a specific document. Please also note that the fee amount to be paid also depends on the complexity of your request.