In their confrontation aboard the Death Star, Obi-Wan addresses Lord Vader thusly:
Only a master of evil, Darth.
Treating it more as a name, than a title.
DVK's answer to the question The use of 'Darth' as Vader's name, not title, in Star Wars shows that, originally, we had Darth Vader being a much less menacing figure, but with that name. So we can be fairly sure the original intent was not for that name to be a title meaning Sith Lord.
I don't recall anyone else in the original trilogy being referred to as a "Darth", so was that being a title something that came later (perhaps appropriated from the EU/Legends), or even the mention of the Sith?
So, when did Darth stop being Vader's first name and become a title?