If a girl wants to be friends first, how do you transition from friend to eventual boyfriend?

So this girl I've been sorta dating is a real slow type of girl. She wants to be friends first and then see where things just naturally go from hanging out, spending time together, etc. I know people say that if you get stuck in the friend zone your doomed, but for this situation there is no other choice. I've been told by her brother and others that's just the type of person she is and I like her enough to give it a shot. What I'm wondering is at what point should I try to make it more, to attempt the crossover from friend to boyfriend or at least friends to dating... I've gone out on a couple dates with her in a month but we're both really busy and I know we both like spending time with each other. I just know for a fact she doesn't like to be rushed into labeling things...
If a girl wants to be friends first, how do you transition from friend to eventual boyfriend?
Post Opinion