how much time is needed for preparing for IAS exam after graduation and what subjects need to be stuidied?

This would depend upon the individual's capacity to learn and grasp and his/her existing knowledge level. Still atleast a rigourous study of 8 to 10 months would be essential to cover up all the aspects, I think.

dear jaspreet, ias is parhaps the most prestigious job in india, so there is a very tough competition. time limit can be determined to clear it. some people start preparing for it even when they are in 10th or 12th. if you have completed graduation, i think deep well planned one year study is enough