Well be honest with yourself. Do you truly think that she might have feelings for you, or that its even a possibility? If so, then go for it right away before someone else does.
I mentioned the girl I liked when I started college. I told her I liked her twice shortly after we first started talking. The first time, she still kept talking to me, but she said she had just gotten out of a relationship so she wasn't ready. The second time, she told me she still had "false hope" that her ex-fiance would return to her, since she was heavily involved in a church at the time and he had left because of drug-related issues. Ironically, her other ex-boyfriend showed up at the church the same day I decided to go with her, and we pretty much stopped talking after that.
A year later, I started having feelings for her, but I knew that even thought we were better friends now than before, I knew she didn't have feelings for me, so I did the stupid thing and decided to wait in hopes of her changing her mind. Turns out, when she found out I liked her, we stopped talking and she hooked up with some guy right after that. But I realized that even though I was scared of getting hurt since I knew it was a definite no, I still should've been honest from the start, that way I could've moved on sooner.