I had the same problem. I liked her alot, and I really wanted to be with her, but I didn't want to lose her as a friend. Well I told her how I felt and she didn't want me whatsoever. She still wanted to be friends but I kinda lost it for her after that. Later I realized I really didn't need her as a friend that much anyways. Friends come and go. If you want her bad enough, no matter how long you wait, it will drive you crazy if you don't tell her how you feel. I'm sure she won't be the one to break up the friendship, she will probably want to remain friends regardless. So just ask her out.