Somewhat off topic, but related. I just checked out the official Dresden Files timeline and saw something interesting. Death Masks; when Maggie was conceived, took place in late February, about 3 years after Storm Front. Storm Front took place in March. That means Maggie was (most likely) born in late November, approximately 3 years and eight months after Storm From. Peace Talks and Battle Ground take place in July, approximately 14 years and several months after Storm Front. Maggie is coming up on her tenth birthday, about the time when she should start to show noticeable magical abilities.
Seeing as the next book will take place during the twelve months following Battle Ground, I think Maggie's magical ability will manifest and be noticed by Harry; and at least one other, most likely Lea, near the end of those twelve months. Near the end of the novel, I think Harry will be forced to make some hard choices regarding Maggie's future education, both regular and magical.