Hey man, im in the EXACT same situation right now. The last thing you wanna do is overstay your welcome. You always want to be the challenge, even when she has a boyfriend.
Tone it down a bit, dont text her as much. Let her think your out with beautiful women having lifelong memories with them while she's tied down with that loser.
Best thing you can do is stay low on the radar and just be there from time to time when she needs someone to talk to. DONT BE AFRAID TO BE THE FRIEND. Trust me on this one. I hate to break it to you, but the chances are slim to none of you getting any while things are going good with her guy.
Let her know that you dont "care" for that relationship. I, personally, dont like the theory of building up the boyfriend. Thats psychological bullshit that'll only make you look like a chump. Because guess what? When she has the inevitable fight with her boyfriend, what's she gonna do on those lonely nights, pull out her vibrator all alone or get a chance to fuck the guy that hates her bastard of a boyfriend?
You're in a beautiful, wonderful scenario if she wants to fuck you. Just be patient. Overplaying your cards and pressuring her will only drive her away and make you think your there for one reason.
In the end, it's HER decision to fuck you or not. When she has that fight, she doesn't wanna run to the guy thats only looking to screw her. She wants to run to the guy she can relate to and can listen to her, and understand her. That's the guy that fucks the girl in the relationship, the confident, cool one that waited patiently for her to come to him. Not the guy texting her everyday with dirty thoughts.
In short: Be cool! Dont text her nearly as often, let her wonder what you're doing, and be vague in your answers if she asks. When you do text her, be fun, flirty, and sexy. But also listen. If she has a problem she wants to talk about, dont hesitate to listen. Eventually, she'll be talking about that fight with her boyfriend. Followed up by a simple, "Well, come on over and we can talk about it." If you cant get laid in THAT situation, you may like boys lol.
I like to call it, "Out boyfriending the boyfriend." Like becoming the model that she wishes her actual boyfriend was. All the while she knows that model is out having a wonderful time without her, eating her up slowly.
Women in relationships aren't nearly as receptive to usual PUA tactics as single women on the prowl. They're looking for that guy that cares about whats going on in their lives. To have someone fill that gap that the boyfriend is clearly ignoring.
Keep us posted man! Good luck
CHAT