In most cases, a lawful permanent resident is eligible to apply for naturalization after having his/her green card for 5 years. Some individuals may be eligible after only 3 years. Still others, including some military members and their dependents, may be eligible immediately after receiving their green cards.
The number of years someone has been a lawful permanent resident is only 1 factor to determine when, and if, that person is eligible for naturalization. Besides meeting the time requirements for having a green card, lawful permanent residents must also satisfy residency requirements, both physical presence and continuous presence. In certain exceptions, time spent outside the U.S. may qualify as actual time spent in the U.S. in order to meet these two presence requirements. Also, lawful permanent residents must prove that they have good moral character in order to become a naturalized U.S. citizen.