The answer to this question depends on how you obtained your lawful permanent resident status. You will be able to apply for United States citizenship either after three or five years of being a legal permanent resident.* If you obtained your green card through a United States citizen spouse, you may apply for United States citizenship after three years of being a legal permanent resident. If you did not, you will be eligible for United States citizenship after being a green card holder for five years.
*Some applicants for citizenship are eligible to apply for citizenship 90 days before the three or five year lawful permanent resident requirement.