When they’re Counsellors of State?
Currently the four Counsellors are Charles, William, Harry and Andrew. When Charles accedes the throne Beatrice will take her place as counsellor, and when William accedes so will Eugenie. Or perhaps they will sooner than that given that Harry and Andrew can, in reality, not represent the Queen any more.
Will they become full time royals at that point? Currently William and Kate are the only working royals of their generation. I know Charles is keen on a slimmed down monarchy, but William will need support from his own generation in the absence of Harry.