How can I work around a China Visa, multi-entry, max stay is 30days, if I want to stay in China for 2 months?

I already possess an active multiple entry China Visa and would like to visit my bf for 2 months. Does any one know how much time is required to between the max consecutive 30days? Or say I stay in china for 15 days, and then travel to Thailand, when I return to china does the 30 days start over?