I might be wrong with the date but to the best of my knowledge Charles X extended the HRH to all male dynasts in 1825. This benefited the Duke of Orleans who up until then had only been HH while his wife as the daughter of the King of Naples was HRH so when visiting the Tuileries the Duchess would be going through doors (portes brisees) with both doors open and the Duke followed but for him only one half of the door remained open and the other half was slammed closed.
Charles X elevated the Orleans line and the last prince of Conde to HRH because so many new kingdoms had been created with lots of HRH's and he felt that the members of the ancient French Royal House needed to be equal to those new reigning dynasties.
It has been confirmed on this board that titles and styles are unlimited in the dynastic male lines, in all German houses (and several others as well). Case in point: HRH Princess Amalie Isabella of Bavaria (1921-1985) was only a great-great-granddaughter of King Ludwig I. Despite the fact that her nearest sovereign ancestor in the house of Wittelsbach was four generations removed from herself, she was still styled with the full royal title and qualifying rank. After all, her ancestry was completely royal within those generations: in particular, all the princes of Bavaria in her paternal line married equally.
Anyhow, I was wondering about the royal who was the furthest removed, in terms of descent from a sovereign ancestor, who was styled with the full title and qualifier. Perhaps it's difficult to pinpoint that person, since duchies, grand duchies, and principalities could all be sovereign houses.
So for practical purposes, perhaps one should restrict discussion to royal dynasties. Has there ever been, for instance, a person who was only a great-great-great-grandchild of a sovereign, but who nevertheless qualified as a prince(ss) and a Royal Highness (entitled thus by virtue of dynastic male-line descent)?
If you look at the various European genealogies, you'll notice that in many cases, after just two or three generations, many minor members of royal families married morganatically.
I may be mistaken, but I’ve heard somewhere that the style of Royal Highness was extended to all members of the French royal dynasty... although the title of Prince of France is informal at best, they all have the status of Prince of the Royal Blood of France (prince du sang).
Putting aside for a moment their descent from Louis Philippe of the French or their claim to be heirs of the Count of Chambord, members of the House of Orléans have Louis XIII as their nearest French king in the male line.
If the Legitimist Royal Highness extends to all Bourbons, the most distant will probably be found among the Orléans.