How often do you need to take vitamin B12 shots?

when i was getting them from my doctor i got them once a month. i just ordered them on line and it said one to two a week. i know that when i was getting them once a month, after about 3 weeks i could feel it "wear off". now i just listen to my body. i am using them for energy so when i start to feel the energy "wear off" i give myself a shot. it comes out to about every 3-4 weeks.