When the text is red counting down, it means Ixion can't spawn at all. Once that counter hits zero, it changes to green and starts counting up. During this time, Ixion will spawn when it feels like it. If it hasn't spawned yet and the counter makes it all the way up to 36, Ixion will be forced to spawn. If you add the two periods of time together, you have a minimum of 36 hours and a max of 72 hours between spawns.

So basically, if the time of death is accurate, don't waste your time waiting in the zone when the counter is in the red phase.