When will i stop bleeding?

I am on the depo provera jab (the contraceptive injection) which lasts for three months at a time. In the first three months i experienced no bleeding whatsoever, which was great. Then in the week running up to when my next injection was due i started to get very light bleeding (spotting). I had my second injection a couple of weeks ago and the bleeding still hasn't stopped, which means i've been spotting for three weeks. This pretty much defeats the object of being on the injection because me and my partner haven't been able to have sex because of it. I'm confused because i had no bleeding whatsoever during the first two months of my first injection. I just want to know if the spotting is going to stop and when?