Lawful permanent or conditional residents should apply for a Re-entry Permit if they intend to be outside the USA for more than 1 year, but less than 2 years. However, if you plan to stay outside the USA for less than 1 year, you will need only your valid green card as a travel document.
After you have submitted Form I-131, Application for Travel Document, you should avoid departing the USA until the Re Entry Permit is granted to you. To file Form I-131 and obtain a Reentry Permit you must be physically present in the United States. Besides, you will have to attend a biometrics appointment that is conducted in the USA. If you travel abroad before providing your biometrics, USCIS may deny your I-131 application.
Thus, it is advisable to submit Form I-131 within 3-4 months before you intend to travel abroad. Meanwhile, in certain extraordinary situations USCIS may employ expedited processing and grant you the travel document sooner. For more information on the subject please read our article “How to make a request for expedited processing of a travel document”.