When a U.S. citizen and foreign-born person decide to marry, they often assume that U.S. citizenship will follow almost automatically. That's far from the truth, unfortunately. Let's take, for example, the situation of a foreign student in the U.S. whose boyfriend, a U.S. citizen, has just proposed. The couple plans to get married and settle down in the United States. How long will it take before the immigrant gets U.S. citizenship?
First, let's get one important thing straight: Marriage to a U.S. citizen makes someone eligible for U.S. lawful permanent residence (a "green card"), not for U.S. citizenship. (At least, not in the short term.) Having a green card for a certain number of years can make the person eligible for U.S. citizenship. But it's a two-step process, at a minimum. In short, even if the U.S. citizen husband sponsors the immigrant, it doesn't turn the immigrant into a U.S. citizen right away.
Below are the steps you will probably have to go through if this situation applies to you, and the typical amount of time each one will take.
NOTE to anyone who isn't in the exact same situation: Procedures differ for applicants who are living outside the U.S., or who are inside the U.S. after an unlawful entry, or who are marrying a U.S. permanent resident rather than a citizen. See this chart for the various procedural scenarios.
If the immigrant in this example remains married to and living with the U.S. husband for three years from the date of your approval for conditional residence, it's finally time to apply for U.S. citizenship (naturalization). The immigrant will need to meet all the eligibility criteria, including being able to speak English and pass an exam covering U.S. history and government.
Someone whose spouse became a U.S. citizen after the immigrant got U.S. residence would have to wait three years from that point to take advantage of the exception, with the result that it might not even help them.
The application for citizenship is made by submitting Form N-400 to USCIS. This part of the process is also a lengthy one. The applicant will have to wait months before being called in for a naturalization interview at a USCIS office, and then after approval, attend a swearing-in ceremony.
By the way, this three-year rule represents an important exception. Most people must wait five years after getting a green card to apply for U.S. citizenship. But the immigrant will need to have met all of those conditions for the entire three years, namely that they were living with the U.S. citizen spouse the whole time, and that the spouse was a U.S. citizen that whole time. In fact, they must remain married all the way up to the swearing-in ceremony to qualify for the three-year exception.
If the marriage breaks up or the couple stops living together, the immigrant must count a full five years from the date of approval for residence to apply to naturalize. The news is also bad if the marriage ends due to the death of the U.S. spouse, as described in My U.S. Citizen Spouse Died: Can I Still Apply to Naturalize After Three Years?.
For comprehensive information on the green card application process, see Fiance and Marriage Visas: A Couple's Guide to U.S. Immigration. Or, if you have any questions about complicating factors in your case, or wish help with the application process, consult an immigration attorney.