I was on the shot for several years. Usually it takes a month for it to become effective, so unprotected sex during this time is not reccomended. I'll say I could NEVER track my period when I was on it, and from day one it was all over the place!
I was on the depo shot from 2008-2010. I NEVER had a period while on the shot. And I did ask my Dr how long to wait as far as having unprotected sex. I was told to use a condom for about 2 weeks then I should be fine. I think it is very common to go without a period while on depo. A lot of my friends that also got the shot also didn't get their periods. Hope this helps.