Validity of S2

Hey,

I have a question concerning my Visa.
I am currently on a S2 type in China (number of entries 2; duration for each stay 90 days). I am wondering whether that means that I need to leave the country after 90 days and be able to re-enter PRC for another 90 days with the same S2 or does it mean that I am allowed to leave the country within 90 days?
So basically is the S2 valid for a period of in total 180 or 90 days?

Thank you very much in advance