How long should you wait?

[glatt]

If she's "not hugely upset" about it, don't wait to tell her how you feel. Maybe give it a few days or even a week, but any longer and you risk sitting on the sidelines again while she starts up with another guy.

Maybe she's been attracted to you all along but never did anything because she's been in a relationship.

Of course, maybe not. Only one way to find out.