It depends on how you acquired your green card. If you qualified for a green card through your marriage to a U.S. citizen, they you are eligible to apply for citizenship after 3 years from the date your green card was issued. If you qualified for a green card in some other way, such as asylum, EB-5, L-1, etc., then you must wait 5 years before you can apply for citizenship.
Answered on Jul 25th, 2014 at 11:32 PM