I found this on a website
As a permanent resident or conditional permanent resident you can travel outside the United States for up to 6 months without losing your green card.
Staying outside the United States for more than 6 months but less than one year will subject you to additional questioning when you return to the United States but you are not required to have a Reentry Permit and shouldn't have any problems getting back in.
If you intend to stay outside the United States for a year or more you will need a Reentry Permit. You apply for the Reentry Permit on Form I-131 Application for Travel Document. You can't apply for a Reentry Permit while outside the U.S. so make sure you get your I-131 submitted before you leave the country. The Reentry Permit is good for up to 2 years and may not be extended. If you are a conditional permanent resident your reentry permit will expire on the same date you are required to apply for removal of the conditions.
