To maintain your LPR or Conditional Residence status (i.e. green card)- you have to keep from abandoning your residence in the U.S. You can have a trip out of the U.S. greater than 180 days without abandoning your residence. You can apply for a re-entry permit, which allows you to stay out of the U.S. for up to two years without abandoning your residence.
You can apply for a reentry permit (on form I131) before you leave the U.S. You can depart before the reentry permit is approved. With such a reentry permit, you can also return to the US at any point until the reentry permit’s expiration date. Reentry permits are issued for two years. At the expiry of these 2 years you can apply for a new one. The second such reentry permit will be granted for two years ago, but then onwards all subsequent ones may only be approved for one year at a time.
If outside of the United States for two or more years after being issued a reentry permit, a Returning Resident (SB-1) immigrant visa is necessary. While outside the United States, permanent residents are still required to file U.S. income tax returns, and failure to do so may jeopardize their status. Permanent resident status will be considered abandoned if a green card holder moves to another country and no longer intends to reside in the United States.