美国绿卡有效期 10 年，10 年后满足要求可以换取新绿卡，过程中也可以选择入籍。绿卡持有者享有当地居民权利，可以在美国合法工作，学习并永居居住，但绿卡持有者同时也需要满足居住要求才能维持绿卡状态，也就是所谓移民监，美国移民监是相对宽松的。
Do not leave the United States for an extended period of time unless the circumstances show that your trip is for a temporary purpose (for example, to attend school, take a temporary job, or care for a family member). If you are absent for a year or longer, you cannot use your Permanent Resident Card to enter the United States
也就是美国移民监的要求一般认为是半年一天（一年2天）最合适，除非特殊情况，一次离开美国最好不要超过1年。
Abandonment or loss of permanent residence status
A green-card holder may abandon permanent residence by filing form I-407, with the green card, at a U.S. Embassy.
Under certain conditions, permanent residence status can be lost involuntarily. This includes committing a criminal act that makes a person removable from the United States.
A person might also be found to have abandoned his/her status if he or she moves to another country to live there permanently, stays outside the USA for more than 365 days (without getting a re-entry permit before leaving), or does not file an income tax return on their worldwide income.
Permanent resident status can also be lost if it is found that the application or grounds for obtaining permanent residence was fraudulent. The failure to renew the permanent resident card does not result in the loss of status, except in the case of conditional permanent residents as noted above. Nevertheless, it is still a good idea to renew the green card on time because it also acts as a work permit and travel permit (advance parole), but if the green card is renewed late, there is no penalty or extra fee to pay.
A person who loses permanent residence status is immediately removable from the United States and must leave the country as soon as possible or face deportation and removal. In some cases the person may be banned from entering the country for three or seven years, or even permanently.
美国入籍要求是5年内每年住半年：
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