Exit flight

I want to travel around China for about 6 weeks. So if all I can get is a 1 month visa ( or can I get a 3 month?) Does my exit flight have to be within the 1 month of the initial visa? or can I go ahead and book my return flight for the date I actually want? Note that I'm a New Zealander with residency in Vietnam and that I'm applying in HCMC.