Usually, you must have your green card for at least 5 years before you can apply for U.S. citizenship.

If you are married to a U.S. citizen, you may apply after just 3 years with a green card.

You must have been physically present in the U.S. for at least 30 months out of the 5 years or 18 months out of 3 years for spouses of citizens.

You can actually submit your citizenship application (Form N-400) up to 90 days before you meet the full 5- or 3-year mark.