What is the validity duration of FAA Medical for international flights?
I believe the ICAO regulation you are thinking of is this one:


  Excerpts from ICAO Annex 1- Personnel Licensing
  
  “1.2.5.2 Except as provided in 1.2.5.2.1, 1.2.5.2.2, 1.2.5.2.3,
  1.2.5.2.4 and 1.2.5.2.5, a Medical Assessment issued in accordance with 1.2.4.5 and 1.2.4.6 shall be valid from the date of the medical
  examination for a period not greater than:
  
  60 months for the private pilot licence — aeroplane;  12 months for
  the commercial pilot licence — aeroplane;  12 months for the airline
  transport pilot licence — aeroplane;”
  
  It also indicates the following in a note:
  
  “Note 2 — When calculated in accordance with 1.2.5.2 and its
  sub-paragraphs, the period of validity will, for the last month
  counted, include the day that has the same calendar number as the date
  of the medical examination or, if that month has no day with that
  number, the last day of that month.”
  
  In ICAO Annex 1, 2.4.1.6, the medical requirement for a commercial
  pilot is prescribed. It states:
  
  “2.4.1.6 Medical fitness
  
  The applicant shall hold a current Class 1 Medical Assessment.”
  
  In ICAO Annex 1, 2.5.1.6, the medical requirement for an airline
  transport pilot is prescribed. It states:
  
  “2.5.1.6 Medical fitness
  
  The applicant shall hold a current Class 1 Medical Assessment.”
  
  “6.1.1 Classes of Medical Assessment
  
  Three classes of Medical Assessment shall be established as follows:
  
  a) Class 1 Medical Assessment; applies to applicants for, and holders
  of:  — commercial pilot licences — aeroplane and helicopter  — airline
  transport pilot licences — aeroplane and helicopter  — flight
  navigator licences  — flight engineer licences
  
  b) Class 2 Medical Assessment; applies to applicants for, and holders
  of:  — private pilot licences — aeroplane and helicopter  — glider
  pilot licences  — free balloon pilot licences
  
  c) Class 3 Medical Assessment; applies to applicants for, and holders
  of:  — air traffic controller licences.”


However, ICAO is not involved in enforcement but rather coming up with standards, which would then be enforced (if adopted) by the member country's aviation authority.  

From the FAA:


  There are many differences to be aware of when flying outside the
  United States (U.S.). International Civil Aviation Organization (ICAO)
  rules prevail. Member states follow ICAO guidelines as published, with
  differences noted, in their official Aeronautical Information
  Publication (AIP).
  http://www.faa.gov/air_traffic/publications/ifim/intl_overview/


For example, the EASA (European Aviation Safety Agency) has rules very similar to the FAA:


  MED.A.055  Validity, revalidation and renewal of medical certificates 
  (a)  Validity  (1) 
  Class 1 medical certificates shall be valid for a period of 12 months. 
  (2) 
  Notwithstanding to paragraph (1) the period of validity shall be reduced to 6 months for pilots 
  who:  (i) 
  are engaged in single­pilot commercial air transport operations carrying passengers and 
  have reached the age of 40  (ii)  have reached the age of 60
  (http://easa.europa.eu/rulemaking/docs/npa/2008/2008-17/NPA%202008-17c.pdf)


However, according to this report: http://dspace.mit.edu/bitstream/handle/1721.1/35725/FlightOpsICATfinal2.pdf,


  Air carrier operations are generally regulated by the country of
  registration and the sovereignty in which the operation is conducted,
  including the Federal Aviation Administration (FAA) in the USA and the
  Joint Aviation Authority (JAA) in Europe. International flights may
  also fall under the jurisdiction of ICAO (International Civil Aviation
  Organization) when operating abroad (U.S.-based carriers are governed
  by both FAA  and ICAO regulations when conducting international
  operations.)


So you might be legal until you hit the international waters.  At the very most you might be able to go abroad, but then not be legal to fly back.

The National Business Aviation Association (NBAA) recommends that pilots comply with the strictest interpretation of the law (so in this case, have the appropriate compliance with ICAO standards) to avoid having to explain the nuances between the two sets of regulations. 
(http://www.nbaa.org/admin/personnel/medical/medical-certificates.php)