Many factors affect b12 absorption1,4 such as gastro intestinal issues, alcohol consumption, genetics and how you take b12 (oral, sublingually, injection).
Use this as a rough guide only to see if you're way out of whack with what you're taking now and see your doctor if you are concerned.
See bottom of page for source material
NOTE: Overlapping (saturation) doses are not considered. See below for more information
NOTE: Err on the side of caution as there is currently no evidence to suggest large doses are detrimental
The chart below describes the absorption rates of b12 based on the referenced data at the bottom of the page.
As you can see b12 absorption drops off drastically after 1mcg and then tends towards about 1% of the dose. In simple terms
the receptors that absorb b12 get saturated at ~1.5mcg2 so any (oral) bigger than that passes along the gut and
is absorbed (if at all) by passive diffusion until the receptors are ready to accept more (about 4-6 hours)3.
Sublingual absorption is different and I have yet to find much comparison data except that which suggests they are similar in correcting deficiencies5.