I already responded to this on the Discord, but I’ll expound a little here.
These are the current spawn rates for the chieftain spawn (this list is actually generated from Xevyr’s Purge Mod – it is a handy tool and isn’t even the point of the mod 
):
Anyway, given that you want to collect all ten chieftains, the average number of chieftains you will need to grab is:
10 * (1 + 1/2 + 1/3 + 1/4 + 1/5 + 1/6 + 1/7 + 1/8 + 1/9 + 1/10) = 29.29
But, since the odds of finding a chieftain is only 5%, you need to multiply that by 20.
So it will take an average of 586 unprovoked assaults against New Asagarth’s leadership to abduct at least one of each chieftain and you should wind up with about 19 duplicate chieftains by the end of your rampage.
Given their 15-minute respawn, if you spend a couple hours instigating eight of these hostilities each day, it should take approximately 74 days of bloodshed and concussions to complete your chieftain collection.