There likely is something of a sustainability deadline, though I suspect that the RNZN might be getting forced to try and push past it a bit. IIRC the current RNZN plan calls for their ANZAC-class frigates to be replaced sometime in the mid-2030's, though I have not heard a specific year target. IMO though, that is pushing things as it is, since the youngest RNZN frigate, Te Mana, will have reached 30 years in commission by the end of 2029. Replacement in the mid-2030's would potentially mean that the RNZN frigates will be approaching 35 years of service by the time they are replaced, and about 15 years after their major upgrade. This could be a problem if something significant kicks off somewhere and NZ has to become involved.