How long do you wait before you should start to worry? If you listen to the fake cops in Hollywood, it's 48 hours. Ask a rock star and they'll tell you it's somewhere around the two-month mark. Ask your mother and she'll probably say 10 minutes. The point is that there's no hard and fast rule - if you haven't seen your girl in a while, you'll just have to listen to your heart.
Of course, if you haven't seen her in a month you can safely assume that it's over. Even if it's just a few days at a time with no word and no explanation, it's still looking a little suss. Not that you have to know where she is at all times. But if she's the type that's glued to her phone, and you're not getting any of the updates? Put up those "Missing" posters.