It is possible that the agency will promptly release all of the information you are seeking. But it is more likely that you will receive one of the following responses:

The agency tells you to wait
Government agencies are required to fill your request within 20 working days unless they qualify for an "unusual circumstances" extension. An agency may take 10 days to decide whether to grant expedited processing if you have asked for it. Many agencies meet their deadlines, but others are notoriously slow, occasionally taking years before a final response is made.

An agency may initially acknowledge one or several of the following things -

Your request has been received and is being processed

A search for the records you've requested has begun

The fees for your request are being calculated

Some agencies may warn you that there is an administrative backlog of requests, and ask you to wait your turn. If you experience a wait, here's what you should do -

After you receive your wait number, call or write the agency's FOIA office and try to get them to commit to a release date for your information. Tell them that if there is no action by this date, you will interpret their "non-denial" as being an outright denial, and you will appeal.

Whatever the cause of the delay, don't hesitate to inquire about the status of your request or about the agency's filing and search procedures in general.