Depo-Provera

6 weeks postpartum, I got my depo shot at home yesterday (dr said I could have a family nurse do it since I used to be on the shot and had no negative reaction). I haven't had a period yet, I stopped the after birth bleeding about 2 1/2 weeks ago. My question is, how long until the shot is effective? I don't remember how it is since it's been a long time since I started it the last go around.