Cattle produce their own B vitamins in the rumen and as a rule don't require supplimental injections. However it can be beneficial in an animal who is sick or has a taxed immune system.
The bottle should have reccomended amounts, somewhere around 3 cc's on the bottle I have.
A B12 shot won't 'pick-up' an otherwise healthy animal as it might in a human. That being said, there are other minerals needed to allow the rumen to make B vitamins. One if I remember correctly is cobalt. Cobalt is found in salt, so make sure you have salt, with cobalt, available at all times.
Jason Trowbridge Southern Angus Farms Alberta Canada