@turkay1-- There is no exact rule about cortisone injections. You can get them as frequently as your doctor sees fit but most doctors like to wait three months for the next injection.
If your first cortisone injection doesn't work though, what makes you think that the second or third one will? In my opinion, cortisone either works or doesn't. I also know from experience that getting cortisone injections at the same part of the body over and over again is not a good idea. It can cause muscle weakness and tissue damage in the long run.
At the end of the day though, it's your and your doctor's decision.