I was on the Depo shot on two different occassions. The first time I took it, I had three periods before they ceased. So, they ceased shortly after the second shot. I was on the shot for one a year, I believe.
I went through a long period of celibacy after a breakup and went off the shot. The second time I got on the shot my periods ceased one month after my shot.
It's different for different people. *shrug*
Heck, some people experience uncontrollably bleeding on the shot, the complete opposite of what one would expect.