If a green card holder has been married to a U.S. citizen for at least three years, and lived with their spouse that entire time, they can apply for U.S. citizenship at least three years after obtaining their green card. They also must have physically lived in the United States for at least 18 months (one-and-a-half years) out of those three years, and their spouse must have been a U.S. citizen for at least three years.
Learn more with our naturalization guide.