Can I ask what prompted having her move out last time? It could be you're still a little upset about that situation.
Once you're an adult, legally able to live on your own, you either move out, or you help out.
Does your son help around the house? Do you ever eat together as a family? Does he do the dishes? Are you still doing his laundry?
What about when the gf visits? What does she do? Does she clean? Do dishes? or do she and your son exist as separate entities from you and just do whatever they like?
Once you've answered those questions, ask yourself what you want/expect. Do you want them to pay board? Do you want them to help around the house? Are you okay with so many days? (I personally wouldn't be). Or ultimately do you want her to stop staying over so often and instead they save to move out on their own?
Bear in mind whatever you do or say WILL rock the boat. When they're old enough to understand, they will. Guaranteed neither she nor he would like it if once they move out you popped over and stayed for that many days EVERY week with them. Always different once you experience it yourself!
It seems to me like you want to tell your son that while it's great that he is serious about someone, you aren't comfortable with the current arrangement. They can save up to move out if they want that next step in their relationship, rather than imposing on you. Just because he's your son doesn't mean you are REQUIRED to let her, or him for that matter, live with you once he's an adult. My brother freely admits he can't afford to move out but it doesn't improve his behaviour!
My brother, 25 y/o, still lives at home with mum. While I call it "home" it isn't my home, it isn't his home, it's my mother's house. He eats whatever he feels like, leaves lights on, shower's twice a day with 2 towels per time (he has short hair!), invites friends over at all hours of the day... and that's just the beginning! He's totally disrespectful of HER house. She's the one paying the bills and yes he pays board, but that doesn't mean he's allowed to do whatever the hell he likes. Board is just to subsidize his costs now he's old enough... and it doesn't cover what he actually "spends".
Most parents want their kids to feel welcome at the family home, but there is such a thing as TOO familiar and that's how my brother is now and I think that's the way your son and his gf are headed