Re: vitamin b12 deficiency how long will it take to get better. I just posted some new results. My b12 was 311. Doc said it was normal (in the normal range) and in six months of taking sublingual tabs two times a day (1000) it went up to 914.e: vitamin b12 deficiency how long will it take to get better. Steph--Your B-12 is where it should be. I run between 800 without supplements and 900 with. I have read twice lately where people have said that their doc wants them to be over 800 not 200 that we all have heard so many times.