This is where it is always a trick thing. It depends on who you are working with.
Best case scenario: She doesn't care
Bad scenarios: If you tell her too early in the relationship, she might not understand and it could become a problem
You wait too long into the relationship to tell her and then she feels like you were holding that back from her and she'll get mad that you were lying (or something along those lines)
So really, it depends on what this girl is like. Try and ease your way into telling her (drop hints for a while) and then find out her views on bisexuality and whatnot, and then tell her. But whatever you do, don't hold it back forever...