It depends.
According to the Interoffice Memorandum from the USCIS (issued 06/01/2007), the "standard timeframe" for an RFE response is as following:
To submit initial evidence that the form requires the applicant or petitioner to file.....30 calendar days.
To submit evidence that Form I-539 (extension of stay or change in status) requires.....30 calendar days.
To submit evidence available in the United States regardless of form type.....42 calendar days.
To submit evidence available from overseas sources regardless of form type.....84 calendar days.
It seems most members of this forum got 12 weeks for their EB-1 or NIW RFEs, but I only got 6 weeks (NIW/TSC, by the way).