Lawful permanent residents or conditional permanent residents who wish to remain outside the United States for more than one year, but less than two, require a re-entry permit.
Trips of less than one year usually do not require any re-entry permit; permanent residents who remain outside the United States for more than two years usually must obtain a “special immigrant” visa at a U.S. consulate abroad, in order to re-enter the United States. You should review your travel plans with the Department of Homeland Security or an American Embassy office to confirm your situation.
(You should note that an absence of more than one year normally will break the period of continuous residence required to become a citizen, even if a re-entry permit is issued.)
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