To answer this question a little back ground is in order. A healthy adult with a well balanced diet never has to worry about Vitamin B12 deficiency. Vitamin B12 is found in all animal based food products, meats, dairy, eggs, fish, cheese and any other product that comes from animals. So in absence of illness that would prevent the absorption of Vitamin B12, deficiency is unlikely.
Vegetarians
Of all the represented groups that do not have a disease, vegetarians run the greatest risk of suffering from Vitamin B12 deficiency. The onset is typically about two years after the person becomes a vegetarian. Some vegetarians never realize the health implications of abstaining from animal products. There are important nutrients that the body cannot function without found in animal products. Supplementing the diet with Vitamin B12 supplements will prevent the Vitamin B12 deficiency. In some cases the vegetarian is faced with a twofold problem, a low ability to absorb Vitamin B12 that was preexisting and the lack of animal products in the diet, which supplies no Vitamin B12 at all. The vegetarian that suffers two fold will very likely face a Vitamin B12 Deficiency.
The onset of the illness will occur in about two years from the time the decision is made to abstain from animal products, a smart medical professional will do a brief history and once it is determined that the sick person is a vegetarian a simple blood test will be taken to determine the Vitamin B12 levels. In some instances the vegetarian will be misdiagnosed because of a too brief background investigation. Treatments will probably include Vitamin B12 shots to regulate the levels. The shots will be given every other day for around two weeks than on a monthly basis.
Disease and Medication
There are a couple of diseases that are linked to the inability to absorb Vitamin B12. Pernicious Anemia is one of those diseases. Pernicious Anemia destroys the cells in the stomach that absorb Vitamin B12, in the case of Pernicious Anemia the Vitamin B12 deficiency is immediate, the treatment is Vitamin B12 shots in high doses which sometimes help and sometimes does not help, the theory is that if the Vitamin B12 is present at very high levels at least some of it will be absorbed. Some folks have very high levels of homocysteine an amino acid (although not actually a disease) that also prevents the absorption of Vitamin B12.
There are some medications that are used for the treatment of heartburn and ulcers that also may cause trouble with the absorption of Vitamin B12 resulting in Vitamin B12 deficiency, typically the onset of the Vitamin B12 deficiency will occur within about twelve months of starting the medication. The medication slowly erodes the body’s ability to absorb Vitamin B12.
Vitamin B12 deficiency usually goes undetected for a great deal of time, many people are misdiagnosed with other diseases, because it is rather rare unless the previous situations listed exist. It is very difficult to pinpoint exactly how long it takes to become deficient in Vitamin B12. There is usually no baseline information to compare the results to, so it is purely guess work.