They have 20 days to respond to the initial request. If they don't respond, the first step should be to submit an administrative appeal to the FOIA contact for the agency. They have 20 days to respond to your appeal. If that appeal is denied or you still don't get a response, you can file a lawsuit in your nearest US District Court (the statue of limitations is 6 years). The agency will respond to the complaint, and you'll know whether it's worth your time to pursue it.