In normal cases Officers are required to serve a waiting period of 12 months from the date of their application (with the exception of Medical and dental (not sure how the scab lifters managed to negotiate a shorter sentance)).
Officers may apply to leave at less than the normal period but that will be determined by the service requirement and the need to avoid undue turbulence.
So, why do the Army and RAF STILL have to submit 18 months notice when the RN only need to serve 12 months?