You may apply for naturalization when you meet all the requirements to become a U.S. citizen.
Preserve Residence for Naturalization
Permanent residents who remain outside the United States for more than 1 uninterrupted year will disrupt their naturalization residence requirement unless they are beneficiaries of an approved Form N-470. You should use Form N-470 if you are a lawful permanent resident who will be absent from the United States for more than one year due to qualifying employment and you want to preserve your residence for naturalization purposes.
Other Issues Affecting Immigrants
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