How long does implantation bleeding last? and is it dark brown? or is that just old blood. i'm on depo shot

Bleeding on Depo: Implantation is actually rare and only occurs when you are pregnant. Since you are on Depoprovera it is much much more likely you are experiencing breakthrough bleeding -- a common and typical side effect. If you think you might be pregnant check then check a home pregnancy test. They are cheap, reliable and available at any pharmacy.