Aaron, on the USN side, numbers in that range predated the implementation of the NATOPS methodology, so 1950's and early 1960's, I'd offer that you would expect that if you did in fact deploy for 18 months. But even by that time, the rotation cycle was typically less than 18 months port-to-port. I seem to recall that Yankee Station rotations were not 12 months continuous, but I'll need to defer to those as were actually on task in that time for their details.
As to the current "one in training, one on task, on in refit" that was a Cold War Optempo intended to deal with the "forever" nature of the Cold War. (It was also aimed at reducing loss of service personnel by leaving/not re enlisting, when 8-10 month deployments were common). I saw that first hand in the early 80's and watched things change over about a decade.