Depo Provera

I barley started using the birth control shot and my doctor gave it to me before my period started. I was told I have to wait 16 days, but so many more answers have been said "only 7 days and it should start working for sure". I wanna make sure its true because I havent started my period but Ive been told too that my periods can be late because of it. Do i have to start bleeding in order for the shot to work? Or am I fully protected after 7 days no matter what?