I remember the statute says

continuously for 180 days or over...something like that.

A suggestion is if you want to stay in China for more than 6 months, and if you have any ligitimate excuse for the stay, then you can leave the green card to the consulate general in China and ask the consulate to keep it until the day you want it back...but you REALLY need a legitimate excuse.

disclaimer: the suggestion is mere hearsay, I myself never tried...I did not know anybody who ever tried...I just recently heard about it.

hope it will help.