If a girl takes a couples of days to text back does that she isn't interested or is playing hard to get?

Probably just testing you. You have to respond less often than she does to pass this test show her that if she doesn't show interest in you you will lose interest in her quickly, because you don't play those games you could even tell her that straight up if you have the balls.

How often does it happen? What are the responses like? Does she read it and not reply or just not read it? All these are major factors