What are the criteria to keep US Green Card valid?

I have US green card, recently quit my job and started a joint venture back home(China) while my wife(also has green card) is still here working in the States. My work can be flexible, but still, I'd like to know if there's a list of requirement out there(like Canada), where Green card holder can make sure that their status will be safe...

so far, I've heard the following unclarified criteria(may not necessarily true...):
the green card holder has to live in the US over 6 months with in a year
or
the green card holder has to live in the US consecutively for 3 months 
or 
reentry time within certain amount ( like less than 10?)

but there is no official site or material ( or I haven't find it) to explain it... so that's why I'm asking...

thanks for the help!!!!!