I love how you answered that question with an entire picture lol.
The picture seems to suggest three units if hair growth, whether those be inches or centimeters is unknown (though based on the subdivisions, inches is much more likely). Working under the assumption that it is inches and Alisons hair is growing at a normal rate, shes apparently been growing it out for ~6 months at the time of this picture. Assuming the length approximately doubled in vol. 5, thatd put her at about a year.
Magic does exist, so nothings truly determinate. However, Mistress magically willed an entire sex change upon her, so if she wanted to grow her hair out faster, she probably couldve done that instantly too. Im inclined to believe the hair growth is natural.
