I started out weekly for 8 weeks then monthly and have been doing that for 2 years. My B-12 was 192 when I started and with last blood work almost 6 months ago I am barely 400. I can tell when I am due my shot by my body not a calendar, and would love to go to every two weeks, but my dr will not let me. I did take a 6 month break to see if my body could maintain my B12 and it could not.
My only piece of advise is whatever you do DO NOT have your blood drawn close after just getting your B12 shot, do it a few days prior to receiving it. It will skew your numbers if you have a new shot in your system.