A proviso that I think is important is that weapons tuition should come from someone qualified to do so.
That might sound obvious but the number of karate and kung fu instructors who 'tack on' some showy weapons training to their curriculum is difficult to estimate.
With the sword arts in particular, for someone, who does not hold grade in them, to add them to an empty-hand course of study is a recipe for less than stellar results. I only have limited exposure to seeing some karateka's and aikidoka's swinging katana around but those I have witnessed would be better advised to seek out an iai sensei.
Given this, I don't know what to answer in the poll
.
My honest answer is 'never' for an empty-hand style with weapons 'trimmings' or right away (clearly
) for a weapons based style.
If the empty-hand teacher is properly graded in a weapons style too, then I would guess that five years of grounding in the non-weapons parts of the schools curriculum should be enough to enable the diversification to be done without dilution of what has been learned.