USCIS has issued a new memorandum which clarifies the 90 day processing rule.  In general, regulations dictate that an application for employment authorization must be processed within 90 days.   If an RFE is issued, however, that 90 clock starts again once the reply is received.  In other words if 89 days have passed before the reply to the RFE is received by USCIS, they can legally take another 90 days from the date the reply is received to finish processing the EAD.