How Many Day Does Birth Control Take To Show Effect Post Its Insertion?

So I got the birth control put in my arm and today makes the seventh day I ve had it. But I wasn t on my period when I got it injected in my arm, because I haven t ha a period in almost four months. So I was wondering how many days does it take to start working and prevent pregnancy. Because my boy friend and I had unprotected sex and I need to know If I can get pregnant or if this thing is already working. Please respond.