How Long Does Depo-Provera Injections Last In The Body?

Hi, I have just taken my first injection of depo provera three weeks ago. I have been bleeding and spotting ever since. I don't like this method of birth control and am worried that it will take a long time to regain my normal 28 day cycle. I wish to become pregnant soon. How long will it take to get out of my system so I can become pregnant?