That is the
Logic Steps Between Air Sessions=20 ;[>0] steps between air fighting sessions, 10*Logic Interval(90sec)
line at the bottom of the file. The "math" in this GWX example is the 20 times 90 for 1800 logic seconds (30 minutes) x 10 = 18000 seconds, or 300 minutes, which is a chance every five hours, based upon the percentages in the lines above. Most airbases have multiple airplanes though, and many bases overlap other bases's territory, so your odds can be rather high of seeing a plane, unless the modders also reduced the number of planes and AirGroups available at some of the airbases..