I haven't read the book, but it is common for people to be diagnosed when certain signs show up (megaloblastic anemia or obviously low B12 serum). Those who are caught early enough will almost always make rapid progress and repair the damage within months.

However, those who are prone to more neurological damage, and are not diagnosed or treated until much more severe damage has occurred, have a much more difficult recovery and the likelihood of much or some damage being permanent.

In those cases, progress can be difficult and occur over several years.

rose