Dear Bismarck,
If the girl already has a boyfriend and you respect her in any way whatsoever, you would NOT ask her out. She's in a relationship, she's probably content with it if she's stuck with him for any amount of time, why are you going to mess that up for her by butting in to where you shouldn't be?
It wouldn't necessarily be good for you to let her know you're insterested. Like I said, if she's in a relationship right now, she definitely wouldn't want to know at all. You're only going to stress her out and alient her more by letting her know, and you're going to destroy any chance of having any sort of relationship with her whatsoever by making things so awkward. If you respect her, you'll be honest with her and not try to "pretend" to not know that she already has a boyfriend. I understand that you like her alot, but what you want to do is pretty selfish and only is in your best interest, not hers.
You have to put things in perspective. You won't meet anyone else you satisfies you if you keep that attitude in your head. No one's asking you to settle or anything, because if you are patient and you look someone else who's perfect for you (and availible) will inevitably show up. But that will take patience and time. There's no point for you now to yearn for someone you simply can't have, and just because you want something strongly doesn't mean you'll ever have it.
There's no way of saying how long you should wait before you ask her out, it completely depends on how she feels about the whole thing, and trying to predict that without knowing her at all is pretty near impossible.
But I can definitely say that a great way to get a relationship going with her is to just talk to her and get to know her and build a friendship with her. From there you can get closer to her and gauge the right time to ask her out. Just be patient and understanding in case she doesn't seem ready.
Cheers
