You will receive a notice of receipt for the form stating that "Your alien card is extended one year - employment and travel authorized". Keep the receipt with your green card. If by some chance you are not approved within a year you will want to make an InfoPass appointment to go to your Local Office to request an I-551 stamp in your passport. This does happen as it sometimes it takes over a year to be approved. After your NOA extension expires, this is your only evidence of legal status which you are required to have, by law. This happens occasionally, that a case takes over a year, but it's rare. Local Offices will not give an I-551 stamp is you have another type of evidence of status (ie, expired Green Card + extension letter).