A Green Card is the physical evidence of the immigration status called permanent residency. With permanent residency, a foreign resident can live, work, study and travel in the United States for as long as they please.
The amount of time that permanent residents (or green card holders) can stay in the United States is truly indefinite as long as they do not violate any of the requirements for keeping permanent resident status.
Long trips abroad may nullify permanent residency status.
However, even letting Green Cards expire will not result in the nullification of permanent resident status!
Currently in the United States there are a great many people (approximately 11 million!) who are in the United States without a proper immigration status.
Public opinion about what there is to be done about this situation is extremely variant and subject to no end of political rhetoric. However, the law currently states that these people should be deported.
Deportation, or removal, is the process of taking a foreign resident from the United States and sending them to another country or region.
The possession of a Green Card prevents the deportation of an individual in all but the most extreme cases. Many of the people who are in the United States without a status, unfortunately, are not allowed to get Green Cards because they do not have a status. It is a bit of a vicious cycle.
Recent discussions in Congress have begun to rethink what to do with these people in the United States. In the very near future there may be a way for these 11 million status-less individuals to stay in the United States and eventually obtain Green Cards.
However, in exchange for this action, Congress will also demand tighter security in the future. This means that all future applicants for entry into the United States must be careful to get a Green Card before they live in the U.S. permanently.