The Department of Homeland Security has recently released a report about the lawful permanent resident population of the United States. One of the findings of the report was that nearly two-thirds of the lawful permanent resident population is eligible for naturalization. In 2009, the Immigration agency estimates that there were 12,450,000 lawful permanent residents in the United States. 7,870,000 of those lawful permanent residents are thought to be eligible for naturalization.
Generally speaking, a lawful permanent resident is eligible to naturalize five years after obtaining permanent residency. However, individuals who are interested in naturalizing should consult a naturalization lawyer before applying to ensure eligibility.