This is a few months late, but for those of you waiting on renewals, this is from the Coast Guard
https://www.uscg.mil/nmc/announcements/pdfs/2016_national_extension_authorization_letter.pdf
My recent renewal took around 3 months and talking to some of my coworkers, that's about what they have been experiencing as well.
USCG NMC:Due to a greater-than-usual volume of mariner credential applications being received, the Coast Guard National Maritime Center (NMC) may be unable to process all Merchant Mariner Credential (MMC) applications prior to the expiration of your current document. In an effort to address this issue, the United States Coast Guard (USCG) is exercising its authority under Title 46, United States Code, Section 7507, and is granting an extension of national endorsements for any mariner whose MMC expires on or after December 1, 2016. This extension is effective immediately and will be valid until September 30, 2017.
https://www.uscg.mil/nmc/announcements/pdfs/2016_national_extension_authorization_letter.pdf
My recent renewal took around 3 months and talking to some of my coworkers, that's about what they have been experiencing as well.