FAQ #1: How long does it take for USCIS to adjudicate a N-400 application for citizenship? Answer: Depends upon the processing center. You can find out here: https://egov.uscis.gov/processing-times/
FAQ #2: How long should it take for a decision under the law? Answer: USCIS has 120 days from the date of the interview to render a decision; when USCIS fails to do so, the law allows an applicant to seek judicial intervention by filing a 8 U.S.C. § 1447(b) lawsuit in Federal District Court. These are typically called mandamus lawsuits.
FAQ #3: If it's been 120 days since my N-400 interview, can I file a mandamus lawsuit, and how much would it cost? Answer: Yes, you can, and they are very effective! We charge $3,000-$4,000 depending upon where petitioner lives, because USCIS-defendant lawsuits must be filed in that jurisdiction, and in some jurisdictions, we would need to procure local counsel to assist.
If you have questions about filing a mandamus lawsuit for an unreasonably delayed decision on a N-400 application, contact: email@example.com.