I am low in b12 and taking 500mcg tablet daily. how long does it take to work?

What type of B12: The most reliable way to make sure you are getting it into your system is by self injection. Next best is by using sub lingual (under the tongue). The 2 best forms of B12 are methylcobalamine hydroxycobalamine. Cyanocobalamine is most commonly available and cheaper than the above 2 and best used by injection I recommend you take 1000ug sub lingual daily or the same dose 1 to 2 x a week