Well, let's say around the time after Marineford, the Gorosei debate whether to send a HK to punish Luffy for attacking the Roswald Family, for example. That would make their first mention around late 2009/early 2010.
So, from there until now, that would be years that plot point of who or what the Holy Knights are. Now, remember other such plot points who have been on the back burner for some time, like Dragon.
Which means it would probably just be a meme of the HK never being able to find Luffy for what would be well over 15 years IRL. Like Dragon became a meme.