Unsure of why the drumbeat is set at this figure.In regard to the Drumbeat (which appears to be approx 2 years or 24mths)
My understanding of the continuous build program had two major points.
First to prevent any future "valley of death" in shipbuilding.
Second to replace each ship after approx 20 years service to do away with the need for a mid life upgrade.
Minor upgrades during the build cycle coupled with refitting earlier builds during regular maintaince would serve to keep the fleet current.
To do this and maintain a fleet of 13 destroyers/frigates requires a drumbeat of approx one every 18 months.
If the first of class (as in the case of the Hunter) takes longer than this to build then the rate of construction may need to be sped up for later units to maintain the expected average service life.