I am interested in knowing whether there is some biologically imposed limit for an amount of B12 that human body can absorb per day. If so, what happens with the rest of the consumed B12? Is it just expelled from the body with other waste?
I don't know if anyone knows of a concrete upper limit on how much can be absorbed. I think the more important point (as discussed in your other question) is that even at low to moderate doses the body doesn't absorb most of the B12 ingested in a supplement. Whatever upper limit there is, to reach it you would probably have to ingest much more than is required to ensure healthy levels of B12 in the body.
B12 is a water soluble vitamin, so whatever the body fails to absorb is excreted in urine.