The 6months is mostly for

The 6month requirement is mostly for naturalization. But even if you don't intend to become a US citizen, you should still be careful not to loose your PR by living abroad for too long.

For under 2 years, you can apply for re-entry permit. And by continuing to file your tax return each year and maintaining active bank/credit card accounts and showing family ties in the US, you can keep your status for the 1-2 year you may be away. But if you intend to live in China for longer than 2 years, then maybe you should consider abandoning your US PR.

http://www.visalaw.com/01jan4/12jan401.html