Hey Rithik, 

You cannot get your documents back untill the admission procedure ends. Original documents are kept as a safeguard that you are enrolled in a particular college to avoid occupancy of multiple seats in various institutes.

Once the semester starts it usually takes 1-2 months to get your documents back. It actually depends on the college administration. Once the session starts you can write an application to the principal for return of documents stating the reason. But before the session, you need to withdraw your admission to get your certificates back.

Hope it helps, good luck!