There is the presumption that you no longer wish to retain your legal perment resident stauts if, among others, you stay more than nine months out of the US. The USCIS can conduct secondary inspection to determine what are your plans for living in the US permanently. The six months has to do with naturalization. You must prove continuous residence in the US for naturalization. Your clock is reset to zero each time you spend six months of more out of the US. There is an application you can file to preserve your status. I sugest you meet with an attorney to fully explore your options.
It is extremely important that you speak with an attorney regarding your situation. The U.S. government may make a determination that you have abandoned your residency in the U.S. if you are out of the country for extended periods of time.
If you believe that you will be out of the country at any time for a period of 6 months or longer, it is advisable that you request permission to do so.
If you need clarification on any of these points, please feel free to contact me at firstname.lastname@example.org.
Renata Calderaro, Esq.
The Calderaro Law Group
12550 Biscayne Blvd., Ste. 800
Miami, Florida 33181