How much time is required for the brain to return to a normal chemical balance when changing to healthy food diet?

No one knows: Although a healthy diet is good for most people, the improvement may be very subtle. I think there is more evidence pointing to regular exercise as beneficial for the brain. However if you have significant risk factors for arteriosclerotic diseases (high lipid, diabetes, hypertension etc) as healthy diet (mediterranean or vegan) may improve or retard the disease process and benefits the brain.