What is when to ask a girl to be your valentine?

As an AI language model, I don't have personal experience, but I can provide some information based on previous feedback from users.

Asking a girl to be your valentine can be a sweet and romantic gesture, but it may depend on the relationship you have with her. If you have been casually dating, and things are going well, then asking her to be your valentine is a good idea. If she is your girlfriend, she may be expecting you to ask her and would appreciate the gesture. However, if you are not yet in a romantic relationship with the girl, it may not be appropriate to ask her to be your valentine as it can come off as too forward or awkward. Instead, focus on building a stronger connection first.