please advice me ,6 mounth 2 entry visa must need exit from china? or no?

sir ,i have visa double entry for 6 mounths,when i need exit from china?and if entry again how long stey in china.accualy my wife is chinese we apply multiple,but immegrition give double entry.i must need exit from china.please can tell me clear? issued date 2011.10.15 valied until 2012.05.14 entry 2
i have to exit from china or no ?
thanks so much.
lama