heads repop every 3 min above 10%. It appears that you have to do a certain amount of damage in order to keep them off (obvious to some)or a certain number of weapon skills(2-3). When you get to 10% heads re-pop every minute.
Seems that its just 1 head at a time and then timer starts over when the other head pops. Meaning each head does not have its own seperate timer.
XI Wiki

