Permanent residents are required to maintain a permanent residence in the U.S. Any absence abroad that extends beyond six months raises the presumption that the Green Card holder has abandoned his or her permanent residence in the U.S. The Green Card holder then has the burden of overcoming this presumption when they attempt to re enter the United States after a long absence. I foresee problems when your parents attempt to return at the end of each absence, unless they obtain permission [in the form of a re entry permit] prior to departure.