The thing is, the B vitamins need to be in balance with each other, so taking a lot more B12 won't give you more benefit. Besides, the B vitamins are water soluble, meaning you will piss out the amount you don't use.
So I would say take the amount recommended by your doctor, if you're doing this at the doctor's suggestion. If not, take the most conservative amount listed on the package directions and increase or decrease the dose to adjust it for your body's needs.