A reentry permit is a travel document that allows a permanent or conditional resident to return to the United States despite a long period of absence of up to two years – depending on the validity of the reentry permit.
In most cases, when a permanent or conditional resident does not obtain a reentry permit prior to leaving the United States and is unable to return for a period of one year or more, the alien may lose his or her permanent resident status or may be required to obtain a returning resident visa before he or she can be admitted to the United States as a permanent resident.
The value of this benefit outweighs the hassle and the cost of applying for a returning resident visa or worse, fighting to keep your permanent or conditional resident status before a US Immigration Judge.
Thus, if you are a permanent resident and you are planning to leave the United States for long periods of time, it is advisable to apply for this benefit before leaving the United States. A reentry permit is generally valid for two years from the date of issuance of the reentry permit unless you are a conditional resident. Reentry permits issued to conditional residents may be valid for up to two years or up to the time the conditional resident must apply for the removal of the conditions of his or her residence, whichever comes first. The alien must return to the United States before the expiration of the reentry permit and apply for its renewal while physically present in the United States.