Ok. Let me explain everything to you. Once she gets U.S. Lawful Permanent Residency she needs to reside in the U.S. to maintain it. Anyone that is a Lawful Permanent Resident (LPR) and is outside of the U.S. for 180 days or more within any 12 month period (not necessarily calendar year) creates a rebuttable presumption of abandonment of residency. That presumption can be rebutted by evidence to the contrary such as filing U.S. taxes, maintaining a home in the U.S. and paying that mortgage or rent, maintaining a U.S. drivers license, U.S. bank accounts with significant movement, etc.
Someone that has been outside of the U.S. for more than 1 year without first having an approved re-entry permit has abandoned their residency and only in very few exceptions (such as serious illness) can they get it back.
Here is an official link:
Since she will need to be outside of the U.S. more than she can be inside of the U.S., she will need to apply for an I-131 Re-Entry Permit. This will allow her to be outside of the U.S. for up to 2 years without abandoning her Residency. At the end of the 2 years, she comes back and can apply for another one for 2 more years. After that they get harder and harder to get and they usually will only issue them for 1 year at a time. So eventually, she will need to start living more time in the U.S. than outside of the U.S. Here is a link to the I-131:
So let's say she does the I-131 and is mostly outside of the U.S. To get U.S. Citizenship through you, she has to have the following:
1) Marriage to you for at least 3 years.
2) Lawful Permanent Residency for at least 3 years.
3) No trips longer than 180 days (which means she needs to come back to the U.S. for a few days every 180 days EVEN THOUGH she has an I-131 Re-Entry Permit, that only protects her against abandonment or Residency. U.S. Citizenship is a different set of laws)
4) 18 months of physical presence in the U.S. out of the 36 months previous to applying.
The fourth is going to be the hardest. So for at least 3 years she should spend 180 days in the U.S. to get her U.S. Citizenship. After that, she can stay in or out, however she likes.
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