This is an interesting analysis and I think the one of the reports used to support the 6 hour ready time as in table V.3 it has an average of 1 to 1.6 sorties a day per aircraft, which if you round up to 2 becomes 12 hours on the ground and 12 in the air for each aircraft, so each flight or the time to ready the aircraft takes place in a 6 hour window.
http://fas.org/man/gao/nsiad98001/a5.htm