A person who has been granted a green card as proof of U.S. Permanent Resident status, must be a continuous resident of the U.S. There are two components to this. The first deals with periods of time outside the U.S. If a Permanent Resident is outside the U.S. for more than six months, but less than a year, there is a presumption that they have abandoned their Permanent Resident status. This can be refuted by showing continued ties to the U.S. such as family, a job, and property in the U.S. If a Permanent Resident is gone for more than a year, they cannot return using the green card. The second component is the long term history of residence. If a Permanent Resident only comes to the U.S. after being gone for 5 ½ months but only stays a month or less, at some point they will be considered to have relinquished their Permanent Residency. To be safe, there should be no trips longer than six months and time spent in the U.S. should be more than time spent outside the U.S.