Multiple entry 10 year visa max stay in China for US citizens

My wife and I, both US citizens, have 10 year multiple entry visas for China. We arrive there on Sep 28 and plan to stay 90 days, leaving on Dec 26. Can we do that without leaving after 60 days and returning? If so, do we go to the Entry/Exit office in Shanghai to get the extension? If not, can we go out, say to HK and then return immediately? How long must we stay out before going back to Shanghai?