What is the icing probability required for an AIRMET?
For any AIRMET, the event must have a probability of 50% at least (that is more than playing playing heads or tails).

Related US NOAA definition for AIRMET Z:


  AIRMETs are issued every 6 hours, where there is a 50% probability
  that any of the following en-route weather phenomena will cover an
  area of 3000 square miles or greater.


Phenomena for AIRMET Z are:


  Icing: Areas of moderate airframe icing, other than convectively
  induced, including the areal extent. Freezing Level information is included after AIRMETs for moderate icing or statements indicating that no significant icing is expected.


Freezing level is defined as:


  The lowest freezing level above the ground or at the SFC as
  appropriate. Freezing levels are delineated using high altitude VOR
  locations describing the location of the lowest freezing level above
  the ground or SFC as appropriate.
  
  Freezing levels above the ground are delineated at 4000 feet intervals
  above mean sea level (AMSL). Multiple freezing levels above the ground
  are delineated using "BOUNDED BY" followed by high altitude VOR
  locations. The range of freezing levels across the forecast area is
  also included.


G-AIRMETs, the graphical version, are issued every 3 hours (but still describe the next 6 hours).