如果沒獲得reentry permit,1年即喪失永久居留權
如果獲得reentry permit,而沒有returning resident visa,2年亦會失效
即使沒標上有效期限,也得要定期使用綠卡回美國,那張才有效

reentry permit者,回美證（白皮書）
http://www.ait.org.tw/zh/visa/iv/IV_info.asp
                                                                               
如果你持有綠卡，但是必須離開美國一年以上，
在你啟程前必須到移民局申請一份回美證。美國在台協會不受理回美證的申請。

Now That You Are A Permanent Resident
http://tinyurl.com/yg62uh
_________________________________________________________
Maintaining Permanent Residence
                                                                               
Maintaining Permanent Residence
You may lose your permanent residence status if you commit an act that
makes you removable from the United States under the law in section 237
of the Immigration and Nationality Act. If you commit such an act,
you may be brought before the immigration courts to determine your right
to remain a Permanent Resident.

You may be found to have abandoned your permanent resident status
if you:

Move to another country intending to live there permanently.
Remain outside of the US for more than one year without obtaining a
  reentry permit or returning resident visa. However in determining
  whether your status has been abandoned any length of absence from the US
  may be considered, even if it is less than one year.
Remain outside of the US for more than two years after issuance
  of a reentry permit without obtaining a returning resident visa. However
  in determining whether your status has been abandoned any length of absence
  from the US may be considered, even if it is less than one year.
Fail to file income tax returns while living outside of the US for any period.
Declare yourself a “nonimmigrant” on your tax returns.
_________________________________________________________

google搜法 http://tinyurl.com/35yxp9