Permanent Resident card issued by the USCIS is valid for 10 years. Green card holders must renew their Permanent Resident card every ten years by filing Form I-90 with the USCIS. The application for your green card renewal can be filed six months before the date of expiration. If you are outside the United States and your green card will expire within six months, you should file for renewal of your green card as soon as you arrive back to the United States. A legal, unexpired Permanent resident status is required for reentry into the United States.
If you are outside the United States when your card has expired and you have not filed for renewal of green card before departure, you must contact the nearest U.S consulate, USCIS office or U.S Port of Entry. It is not possible to renew a green card or File Form I-90 with the USCIS when you are outside the United States.
Abroad on a temporary trip:
If you are outside the United States for less than a year with an expired green card, you may face a problem with your airline carriers or any other transportation means. This happens because they check the passengers for a valid visa to the country they are taking them to. So you need to obtain a transportation letter from the US consulate abroad. To obtain the transportation letter, you need to visit the nearest US Consulate/Embassy, submit relevant documents, including a copy of the expired green card. This transportation letter issued by an overseas US consulate will help you board the airlines and travel to the United States.
After arrival at the Port of Entry, Customs and Border Protection (CBP) officers will interview you and you will be permitted to enter the country only after they make sure that you are a legal permanent resident of the United States. Soon after your arrival in the United States, you have to file Form I-90, Green card renewal application with the USCIS.
Abroad on a trip for more than a year (1-2 years) -- Reentry Permit:
If you are a Green card holder, and your trip abroad takes more than a year, you are required to obtain a Reentry permit from the USCIS before leaving the United States. If the Green card expires while you are abroad, or the Reentry permit expires, that is, if you have stayed abroad for more than a year, you will be considered as having abandoned your residence in the United States by the USCIS, unless you can prove that the overstay abroad was due to unexpected reasons like falling too sick and unable to travel back to the US within the Reentry Permit valid time.
Abroad for more than 2 years -- Returning Resident Visa (SB-1):
In case you have been abroad for more than two years, in order to return home to the United States, you need to apply for a Returning Resident Visa (SB-1). This can be done at any US Consulate abroad. While applying for the Returning Resident Visa, you must show proof that you haven’t abandoned your Permanent Residence in the US. If the consulate refuses the SB-1 visa, you have to file for a new nonimmigrant or immigrant visa to return back to the United States.