The N-400 form is the form that U.S. lawful permanent residents must fill out in order to apply for U.S. citizenship through a process called naturalization. However, timing is important here. With very rare exceptions, people who haven't yet become permanent residents are not allowed to apply to naturalize at all. And once you've become a U.S. permanent resident, you'll need to consider the following timing issues:
We'll cover these in more detail below.
You must be at least 18 years of age in order to apply to naturalize. However, there are situations in which younger people can obtain U.S. citizenship automatically, through their parents.
You are required to have lived in the United States as a lawful permanent resident for at least five years before applying to naturalize -- though exceptions do exist, as described next.
If you got your residence as a refugees or asylee, you will benefit from something called "rollback," which allows you to count some of the time before you actually applied for your green card as if you already had permanent residence. If you entered the U.S. as a refugee, you can count all the time starting on your day of entry, no matter how long it took you to apply for your green card. If you received asylum within the U.S., you can count one year of your asylum status toward your permanent residence. (Note: In either case, your green card will already show the rolled-back date, so you'll still need to count five years from that date.)
Spouses of U.S. citizens, who have been married and living with the citizen for the last three years, can apply to naturalize after three, rather than five years with a green card. And battered spouses who got their green cards based on marriage to a U.S. citizen, but who filed independently (using Form I-360) due to the abuse, can also use this three-year exception.
U.S. military personnel and their spouses (or widows and widowers) can also make use of various exceptions. For example, those who have served honorably for one year and got an honorable discharge can apply to naturalize without waiting beyond the date they get their green card -- but must apply within six months of discharge. And people who served in certian conflicts (including all the major wars in the last century) can apply for U.S. citizenship even without getting a green card first.
You must have lived in the United States for at least half of your five years of permanent residence. Or, if an exception allows you to apply to naturalize in less time, you must have lived in the U.S. for half that amount of time -- 18 months for spouses of U.S. citizens, for example.
You'll need to have not only lived and been "physically present" in the United States for a certain number of years, but not have broken up your residency with any long absences. An absence of six months or more will raise a presumption that you've broken your continuous presence by living in another country. In the worst case, the immigration authorities could decide that you abandoned your U.S. residence altogether, and effectively gave up your right to a green card (in which case you'd be placed in removal proceedings, in immigration court).
Three months is the minimum time in which you need to have lived in a particular state before submitting an application to naturalize that will be reviewed by the nearest USCIS (U.S. Citizenship and Immigration Services) office there.
You must submit the completed Form N-400 to the USCIS center having jurisdiction over your place of residence in the United States. Because USCIS tends to be quite delayed, you're allowed to submit the application up to 90 days before you've actually reached your five years (or fewer years, if you fall into an exception).
There's no upper limit on how long you can wait to submit Form N-400. Many permanent residents live in the U.S. for decades before deciding to become citizens.
Generally the time for processing an N-400 form is five to seven months, though this can vary by region and depending on the number of people who apply at the same time.
After USCIS does a basic review to make sure the application you've sent in is complete, and that you've paid your fee, you will be asked to report for fingerprinting (biometrics). Several weeks after that, USCIS will call you in for an interview, where you'll be examined on your English and tested on your knowledge of U.S. history and government, and the official will decide whether you qualify for citizenship. If you do, then you'll later be called in for a swearing-in ceremony, where you'll actually receive a Certicate of Citizenship.
See Steps to Become an American Citizen for more on what follows.
If you have any doubts about any aspect of your eligibility or the proper timing of your N-400 application, consult an experienced U.S. immigration attorney. The attorney can assist you in all aspects of the naturalization process and accompany you to the interview.