Vitamin b12 1000 mcg, what is the half life on this vitamin and how long until it's out of your system?

No useful half life.: Typical half life does not make much sense since the vitamin is not strictly "metabolized." It is taken up by tissues and converted by tissues to the active form- methylcobalamin. Stored in liver and bone marrow- less so in other organs. Once these sites are full, the remainder is excreted at a rate dependent upon state of hydration, other meds, etc.