I don't think anyone should move a lover into their parent's home! Let's say the couple breaks up. Is that person going to keep using their family home as a "test ground" to allow future lovers to move in and out before they get their own place?
Ideally both people should be (independent) enough to have their own apartments before considering living together.
It also depends on how long the couple have been "seriously" dating. Odds are if you been dating 18 months to 2 years on a (regular) basis spending nights/weekends at each other's apartment you probably have some clothes, toothbrush, and other articles in both places. There should be an indication of whether you can live together and deal with each other's habits/ways over a period of time.
Last but not least (both) people must know (why) they are moving in together. Is it just to save money? Is it for convenience?
Too often couples move in together with one person believing it's a (step) towards marriage. Very often it is the woman who is frustrated when years go by without getting married. If (marriage) is the objective then get engaged first! (buy the ring and send out the "save the date" notices even if it is a year away to family and friends on or before the day you move in). If things don't work out you can always call off the engagement/wedding. The point is (both people) be serious enough about one another to publically announce their marriage intentions prior to moving in.