time frame to reapply after the denied application

Good day. My z type application was denied when i applied last December 2014. I don't have an overstay issue in china. But my passport shows a cancelled visa before i got an F type, some said it is an extension. My school/Kindergarten sent me another original copies of my contract (Chinese and English) after i got denied. May I know how many days or months am i allowed to re-apply? May i also know what number to call in order to provide me the reasons i was denied? Coz i tried to call the embassy but the person said they are not required to provide reasons. I understand but it seems unfair not to know it. There might be someone who can help at least to save my effort and not prolong my agony. Thanks for the reply.