This is not an overnight process. If the error involves a previous credit account or loan, that creditor has thirty days after you have filed for a correction to present its side to the credit bureau. It is not unusual for that time to be extended.
Once all the information is gathered by the credit bureau an investigation is set. There is no time period set for the completion of this type of investigation and there may be delays in determining if the entry was an error. All three credit bureaus have facilities online for you to monitor how your dispute is proceeding.