B12 deficiency is very serious. It causes neurological problems which can be misdiagnosed as multiple sclerosis. Sounds like they caught it before it got that far, but it will still take time to reverse any damage.

Insufficient stomach acid can make it difficult to get enough B12 from food. Vegetarians and especially vegans are often advised to take supplements. I found little tablets that go under the tongue, to be allowed to dissolve. The capillaries there can absorb B12.

Oddly enough I never thought to ask anyone how long it can take to correct problems with deficiency once they start. As a vegetarian I was just concerned with preventing them.