I only had one depo shot because 2 1/2 weeks into it I started to bleed. That lasted 2 weeks and stopped. But for some reason (probably as hali said) when I had sex a week later it started right back up and just never stopped. Hense why I only got one.
one of the reasons I got off it, besides bleeding all the time, which got worse after sex, was I didn't trust what that level of hormones would do to my body with no break. I'm only 16 and even though I don't want any children you just don't want to screw up your chances if you suddenly decide some day you do. I've heard of a lot of health troubles with the shot so I guess I advise you to be careful about being on it a long time.