Its my understanding that it takes a long while for the B12 levels to come down after stopping a B12 suppliment.
The liver stores B12 for quite a while.
What is not regularly used is excreted thru the urine, similar to excess Vit C dosages.
My doc has to note to the lab, that I take a B12 suppliment, or my tests come back as a 'flag' that my levels are 'abnormally' high.
As a matter of fact, my levels, while taking B12 suppliment-
are between 1200 & 1500. Not excessively high, but higher than 'normal'.