When you get a green card or a Legal Permanent Resident, it is a privilege that can be revoked. One method of losing this is when you abandon your permanent residence in the US.
If you stay out of the US for less than 6 months continuously, you will not have a problem in entering the States as a Legal Permanent Resident. However if you are absent for more than 6 months continuously and if you are absent for a continuous year then there is the presumption that you do not want a permanent residence in the States. So when you return there will be a red flag raised, and you would need to prove that you did not want to abandon the residence and would need to explain your absence.
Various factors could influence the abandonment decision and these factors include the reason for absence, the length of these absences, the types of connections which are maintained in the States and the number of these connections.
If you are a person who is traveling abroad for more than a year continuously, you would need to get a re-entry permit so that you can be readmitted to the States. This re-entry permit will be needed before you can even leave from the States and those that are between 14 and 79 need to provide biometrics before they leave the States. This permit is usually given for two years, and this is proof that you don’t want to abandon the permanent residence.
A re-entry permit helps only for proving that you don’t intend abandoning the permanent residence but it does not preserve your residence for naturalization.
If you are a person who wants to be a citizen of the US and yet need to be away for prolonged periods, you need proper assistance and guidance from professionals to help you.