I really don't have a defining line. It's very grey for me. I wouldn't expect someone to take out a HELOC to pay out equity for their girlfriend that moved into a house, that he already owned, for six months. But on the other extreme, a couple that has been living together for five years feels like she should get something. Especially if she helped with the down payment like my GF did.
Where the line between these two extremes is? I don't really have a catch all answer. It'd have to be case by case.