How Effective Are The Vitamin B 12 Injections?

my vitamin b12 labs are 288. I just started taking shots once a month...started last week. seems as though the injection lasts about 2 weeks. Monday of this week I received my second shot. feels like it is just now kicking in. my question would be is 1) What amount of time should the shot be effective and 2) when should it start taking effect after the injection?