Work out how long you actually have left, and the date you need to put it in the mail — not the date it is due.
A Request for Evidence means the officer cannot approve the case on what is currently in the file, but has not decided to deny it either. It is an opportunity, and it is a narrow one: you generally get a single response, and USCIS treats what you send as your complete answer. Sending part of the evidence now and the rest later does not work — the case is decided on what arrived by the deadline.
A Notice of Intent to Deny is a stronger signal. The officer has reached a negative conclusion and is giving you a chance to rebut it before it becomes final. NOID response windows are typically much shorter than RFE windows, which is exactly why the date on the notice matters more than any general rule.
USCIS does not routinely grant extensions of an RFE deadline, and the response period stated on the notice is generally the maximum available rather than a starting point for negotiation. Plan around the printed date rather than around getting more time. If circumstances genuinely prevent a complete response — a medical record or a foreign civil document that cannot be obtained in time — the practical move is to send everything you do have by the deadline, with an explanation of what is outstanding and why, rather than to send nothing and ask for longer.
The most expensive RFE mistake is answering the wrong question — sending more of the evidence you already sent instead of the evidence the officer asked for. FormGuard checks your form and the package you are sending back against the rejection and evidence standards USCIS applies.
Check my response package — $39 Read the RFE guide