Becoming a permanent resident carries with it the obligation to live and work in the U.S. on much more than a casual basis. Trips outside the U.S. are tolerated and it is recommended to return to the U.S. every 6 months but the real test looks to the intent to leave this country. Extended stays overseas beyond 1 year manifest the intent to forfeit the permanent residency. This can be avoided by securing a reentry permit prior to departure. It ensures that your immigrant status does not become invalid. It is processed the same as a travel document or advanced parole.