When you get the Approved in Principle letter/email, you have 6 months to send in your passport for the visa label to be put in. From the date on the visa label, you have a year till you must enter NZ and present your passport at immigration upon arrival to activate your visa.
So in all, it's (just under) 18 months from approval, till you must arrive. However, if you still have matters to sort out in your previous home country, there is nothing to stop you leaving again immediately, once your visa is activated, and coming back later.