“Where do you see this going?”
If you answer with the “long run” response like marriage/kids etc. you could freak her out. (Even if it’s what she thought she wanted to hear! Women, like everyone, can be freaked out when what they thought they wanted is just handed to them.) Just say this, “All I know is that I’m really happy for you and I see no reason for this to end. I plan on staying around for a while if that’s okay.” She’ll love that last line. It will make her feel like the one in power, but not pressured.