Im in the same situ.

my gf is a single mum of 2 and just moved into a house from the council.

Im there most weekends, she was told a maximum of 3 nites a week for anyone else to stay. So we try to keep to that. I dont pay any rent but I do pay towards my usage of food, electricity etc..

I dont keep anything of mine there (i have to pack a bag of clothes every weekend)

If I do move in (which is on the cards) then she will expect to lose her benefit and we will be paying full rent on the house.