USCIS can process your response for RFE in as little as one week, but usually they’ll take 6 to 9 weeks for processing. However, there are examples when they took 12 weeks or more to process the response against an RFE.
Bottom line: No one can predict, how much time USCIS will take to process a response. It is advisable to contact USCIS, if you (read Sponsor) do not receive any response from USCIS in 90 days or 12 weeks of time.