The state of boy/girlfriendshipness must be formally announced, ideally through a public advertisment in The Times, but these days more commonly on the official web-based boards set up for this purpose.
I always understood the situation to become officially acknowledged when, and only when, you were finally caught out behind the bike shed. Or adult equivalent, I suppose, in this case.
I think that it's when engaging in romantic liaisons with a third party would be frowned upon by the other person. Unless they're involved as well, of course. That's an entirely different matter.
Penelope is proud to announce that after many years of searching, she has found a man with sufficiently low standards to happily accompany her for cocktails, noodles, full English breakfasts and participate in the odd snog. [Rab] will that do?