A green card holder can apply for U.S. citizenship generally after meeting the following conditions:

• If you are a green card holder with no special circumstances, you can apply for citizenship after at least 5 years of continuous permanent residence in the U.S. During these 5 years, you must have physically lived in the U.S. for at least 30 months (2.5 years).
• If you are married to a U.S. citizen and have a green card based on marriage, you can apply after at least 3 years of having the green card, provided you have lived with your U.S. citizen spouse the entire time. You must have physically lived in the U.S. for at least 18 months (1.5 years) during those 3 years, and your spouse must have been a U.S. citizen for at least 3 years.
• Some special cases like military service or being a widow/widower of a U.S. citizen in the military can allow earlier or immediate application, but these depend on specific conditions.

Additionally, you must be at least 18 years old, show good moral character, and meet other requirements like English language proficiency and knowledge of U.S. history and government to apply successfully. You may also file your naturalization application up to 90 days before meeting the required residency period to get a head start in the process. These are the general eligibility timelines for a green card holder to apply for U.S. citizenship.