You do not have to obtain a visa from an American Consulate for returning to the U.S. after traveling abroad
You can remain outside the U.S. for the validity of the re-entry permit
Requirements for Re-entry Permit
You should apply for a re-entry permit if you are:
A legal permanent resident and you wish to remain outside the US for more than one year
A legal permanent resident who cannot get a national passport from your country of nationality (some countries accept a re-entry permit in lieu of a passport)
Note: You do not need a re-entry permit if you are leaving the country for less than one year. However, if you leave the country for more than one year, you will break the period of continuous residence that is required to become a citizen even if you get a re-entry permit.
Applying for a Re-entry Permit
You can obtain a re-entry permit by filing an Application for Travel Document (Form I-131) and accompanying documents with the U.S. Citizenship and Immigration Services (USCIS).