Glad it's working for you.
Yes- a big part of it is how much she actually likes you. The advice in this thread is simply how to avoid f**king it up by coming across as needy.
I think in this era of constant communication, girls have become more finely attuned to implicit displays of neediness in males. Whereas ten years ago it was an AFC move to tell a girl that you really like her, today it's an AFC move to text a girl an inside joke the day after a first date, or to add her on facebook after hooking up with her. There is much less room for error.
Even when you text a girl to hang out and she can't because of a legitimate reason, your value drops ever so slightly. The more often this happens, the worse ("negative momentum," if you will)--even though you haven't done anything wrong. To address this, don't respond to any cancellation texts that don't include a counteroffer. In fact, don't respond to any texts that begin with the word "Sorry."
The good news, however, is that if a girl has high interest in you, her schedule will revolve around YOUR availability. A girl that can't meet up because she takes a yoga class on Tuesday nights is a girl with low interest.