As I recall, the time to do so is only available during the storyline when you are looking for a fairy for the guild, and you can only flirt with her if your reputation is high enough with Wimborn.
So best thing to do is spend several days saving travelers from bandits and not asking a reward right after you get the quest for capturing a fairy until Maple flirts with you. I also believe that each level of flirting requires a higher reputation.