Long story short, I moved in with a friend last summer and although she originally said her dp would stay 3/4 nights a week, this has turned out to be more like 6 or 7. She hasn't ever approached it or asked if I'm ok with the change.
He finishes work late so tends to come over at 10 or 11, spend a bit of time downstairs then go to her room - she has a bathroom so he can keep himself to himself (a cynic might say hide!). He then normally leaves after I've left the next day, and spends weekends here too - so goes home once a week if that. To me, that is moving in!
Anyway I didn't mention it for ages but then cracked the other night and politely asked if he had moved in - she was absolutely taken aback and started insisting it was only 5 nights max and that it didn't count as he didn't spend that long in communal areas/he only came here to sleep - but a night is a night, right?! He keeps his stuff here, showers here and she does his washing. Oh and his food is kept here, with hers.
My feeling is that he needs to start contributing to rent and bills then - but who is fair?!