"How often do I need to come to the U.S. to keep my green card and apply for U.S. citizenship?"I have heard this question countless times from lawful permanent residents. Below are the most important considerations for lawful permanent residents with frequent or extended trips abroad.

Avoiding abandonment of your lawful permanent resident status

A common myth among lawful permanent residents is that you can simply come to the U.S. every six months and keep your green card. This is not true. Eventually, U.S. Customs & Border Protection (CBP) will figure out that you are visiting the U.S. rather than residing here. A U.S. lawful permanent resident should reside in the U.S., not visit. Thus, CBP will focus on the number of days you were in the in the U.S. compared to the number of days you were out of the U.S.

Being absent from the U.S. for six months or longer will often prompt CBP questioning and requests for proof of U.S. residence (e.g. taxes, employment records, bills, leases, mortgages). Filing U.S. federal income tax returns as a non-resident is a significant problem.
The reason for the trip (and how you can prove it) often becomes as significant as the actual time spent abroad. If the extended foreign travel is temporary (e.g. short-term employment, caring for sick family member, advanced studies), then having the appropriate documentation to prove this will make your entries into the U.S. smoother. A Reentry Permit may be advisable for some LPR's who need to leave the U.S. for extended periods of time.

Finally, if you are a lawful permanent resident who remains outside the U.S. for one year or longer, consider your green card cancelled by operation of law. Some lawful permanent residents, however, can request a special Returning Resident visa to avoid this fate through their U.S. consulate abroad.