mangesh,
i dont think there is any specific timeline
but, after having missed one test date and getting it re-scheduled apprx 4 weeks later, i still haven't taken my oath and it's a almost a year since i filed for my citizenship (just 2 weeks short infact)
my file at the local miss. center (the worst probably due to a very hgih concentration of immigrations there) has been in processing since march and i still don't have a date scheduled for the oath.
a friend who took the test 3 weeks before i did also have not taken the oath as yet.
hope that helps.