The first step of the process takes slightly more than 14 months, on average, but can be faster or slower depending on your location. Note that the processing time for an N-400 application is from receipt through the oath ceremony, not approval of the application.
Sending your U.S. citizenship application and supporting documents to USCIS kicks off the process. The faster you can collect supporting documentation — for example, a photocopy of your green card (Form I-551, officially called the “Permanent Resident Card”) — the sooner you can send these to USCIS, which can then begin to review your application.
It’s important to make sure that your application is complete, your answers are correct, and your supporting documents are in the format and order the U.S. government prefers. If any of your documents is written in a language other than English, you must obtain a certified English translation of that document, as well — so be mindful of these extra steps, which can take additional time to complete. Any missing information or documentation will likely slow down the process.
IMPORTANT: Make sure to notify USCIS any time you move or change your mailing address to avoid missing official notices.