Depo shot and period

Jennifer
So I got the depo shot just over a month ago and since then I've had one period that lasted longer than usual, though the extra 3 days were more spotting than the full thing. 
My period was due to start today, but it hasn't. I was told that most people who get the Depo shot stop getting their periods altogether, but it might take a while. Is it possible that I'm already at the point where I won't get my period anymore? I thought it would take a few cycles.