As mentioned above, it depends which one you have.
Most brands out there are cyanacobalamin, which means you have to inject it daily, and you should use about 2,000mcg/day.
Synthelamin is the only one that I know off that contains two esters - cyanacobalamin for quick acting (1 day) and hydroxycobalamin for sustained action (1 week).
It's also 2,000mcg/ml so double the strength of other B-12s.
I recomend to inject SQ or IM 1ml ED of Synthelamin to build it up in your body (in case you have a deficiency) and then use 1ml twice a week there after. That way a bottle lasts you 1 year for less then 50 bucks.
The only other hydroxycobalamin I ever found was Neo-Cytamen which costs $10 for 2 x 1ml amps at the pharmacy. That's expensive.