I think that the intention is that everyone who is currently a working member of the family - the children and spouses of The Queen and her cousins and their spouses, except Michael and Marie-Christine, will continue until they choose to retire or they pass on.
However I don't think Beatrice or Eugenie, or in time Harry's children, will ever take on royal duties as their main occupation.
That will slim down the royal family considerably over time.
e.g. 10 years from now it is highly likely that 4 of those who currently carry out royal duties will have passed on or be retired - The Queen (if she is alive she will, of course, be still doing the boxes etc but probably only doing around 150 engagements a year - down from her present total of around 350 - so the daily grind of things like visiting a dance group will be off her list), Philip, The Duke of Kent and Princess Alexandra.
In that time there will only be Harry's wife to add to the current available list of working royals - unless Andrew also remarries. Beatrice and Eugenie won't be working for The Firm and will only be seen at the big events.
That means that the current 15 will have been reduced to 12 in the next 10 years.
Add another 10 years and in all likelihood the Gloucester's will have retired. I am having Charles live into his 90s as an average of his closest ancestors, his parents, and his grandparents - taking into account that unlike his grandfathers he has never smoked or been a heavy drinker. I also don't see Camilla with us then, or if she is I suspect she also will be doing fewer that 150 engagements. The other one who will have reduced heavily by then, I suspect will be Anne. That takes that 12 number down to 8 - 9 as there would have been no new additions - George will only be rising 21.
Assuming William and Kate really are conservationists then they will only have 2 children so in 30 years George will probably be adding his partner and his sibling his/her partner to the mix but by then Anne, Charles, Camilla, The Gloucester's, Kent's, The Queen and Philip and Andrew, Edward and Sophie will all be gone or retired or so reduced they won't really be contributing large numbers leaving 8 at most - the monarch and spouse, the monarch's sibling and spouse, the monarch's children and their spouses while the monarch's sibling's children will be doing real jobs in the real world.
30 years from now I think the total working family will be around 8.