The Livestrong article: How Much Protein to Prevent Hair Loss claims:
Hair is made from protein. When the body does not get enough protein, it rations it by shutting down the production of non-essential protein, including hair growth. Hair loss is noticeable about two to three months after protein intake drops below requirements. Hair growth will begin again once protein intake meets needs.
Is there evidence that hair loss due to protein deficiency is only temporary?