You must be a Lawful Permanent Resident (LPR or green card holder) for 5 years to apply for citizenship and naturalization, or 3 years if the application is based on marriage. USCIS (Immigration) recently clarified the date to be used, which can be confusing. In most cases you become an LPR on the date your application for adjustment of status is approved (if you’re in the U.S.) or when you enter and are legally admitted into the U.S. with an immigrant visa (for those abroad). If you want to apply at the earliest possible date, you can file up to 90 days before the 5- or 3-year dates.
The date you become an LPR is earlier in limited circumstances which apply primarily to refugees and asylees.
