The fact is that no one reacts to vitamin B12 injections in the same way.  Studies have found that how long B12 remains in your system can depend on many things and underlying factors such as tendencies towards developing certain diseases, as well as your lifestyle and diet choices.  These can include food, drinking, medications, drug use and so forth.

Injections bypass the need for the vitamin to be absorbed by the intestines so the body can begin using it within minutes.  Vitamin B12 given through subcutaneous or intramuscular injections is rapidly absorbed and used by the body as needed.  Unless there is a severe deficiency, it can be effective for four to eight days and even longer in some cases.  For those that have a higher rate of metabolism, are more physically active, or are naturally deficient, B12 may be exhausted (used) more quickly.

In general, the body does not store much B12.  However, if you have regular injections there is a better opportunity to maintain some levels more consistently.  Finding your routine and staying on schedule seems to give the best long term results.