What is RFE (Request for Evidence)
Request for Evidence (RFE) is a letter or notice USCIS adjudication officer uses to request additional information for pending applications.

Why and when does USCIS issue RFE's?
Generally, an application or petition falls into 3 categories:
(1) record is complete and case is approvable;
(2) is not approvable based on evidence of clear ineligibility;
(3) its evidence raises underlying questions or does not fully establish eligibility.
When there is a particular piece or pieces of necessary evidence missing, or there are questions regarding the evidence submitted, the USCIS officer will issue an RFE so that eligibility determination can be made.

What should I do if I receive an RFE from USCIS?
You should read it carefully and determine what kind of evidence or document is requested. Some RFE's are more complex than the others; if you receive a complex RFE and do not think you understand what is needed or how to respond, you should retain a competent immigration attorney to address the issues raised in the RFE. Note that only petitioner and/or attorney on the case receive RFE's on employment-based petitions.

How much time does USCIS allow to file a response? Can I just ignore it?
Depending on the complexity of the RFE, it may range from 30 days to 84 days, whatever the USCIS officer indicates on the letter. It is imperative you file the response by the deadline. Late response will not be allowed. If you ignore the RFE, USCIS will make a decision based on the submitted evidence, which generally will be unfavorable.

After I file the response, how long does it take for the USCIS to render a decision?
It varies from a few days to more than 60 days. After 60 days if you have heard from the USCIS you may file call the National Customer Service Center to file a Service Request.