One of the advantages of have a US green card is that, as a permanent resident, you have more freedom to travel to and from the United States. In fact, as a green card holder, you are allowed to leave and re-enter the United States multiple times. In fact, the major restriction on your travel as a US green card holder is that you cannot stay outside the US for 12 months (one year) or longer. If you stay outside the US for at least one year, you may be seen as a abandoning your permanent resident status.
If you plan on remaining outside of the United States for at least one year, or if there is a chance that you will remain out of the US for that long, you will need to apply to the U.S. Citizenship and Immigration Service (USCIS) for a re-entry permit before you leave the United States. The re-entry permit is valid for two years, in most cases, and will allow you to leave the United States for that period of time indicated on the re-entry permit without losing your permanent resident status. It is important to get your re-entry permit before you leave the United States and it is important not to overstay the date indicated on your re-entry permit. Otherwise, you may be denied entry into the United States.
It is a good idea to apply for a re-entry permit if you are a green card holder and will be staying outside the US for an extended period of time. If you will be staying outside the US for 11 months, for example, you may still want to apply for a re-entry permit. That way, if natural disaster or some unforeseen circumstance prevents you from returning as planned, you will still safeguard your status and you will not need to apply for a green card all over again.
You can apply for a re-entry permit by filing USCIS Form I-131 (Application for a Travel Document). You can also file this same form if you have not yet received your green card but need to leave the US for an emergency.
Another thing you will want to consider before leaving the US as a green card holder is the expiry date of your green card. If your green card expires while you are abroad, you may be denied entry into the United States. Therefore, before you travel, you will want to apply for a renewal of your US green card to ensure that your green card will be valid during the entire period you are away from the US. Another thing you will want to consider before traveling outside the US are your future plans for naturalization. If you plan on being outside the US for more than six months, this may not affect your green card status, but it can affect your claims to residency. That is, to apply for naturalization you must generally show a period of residency in the United States with no extended stays abroad.