Is it something you can force on a superior several times?
I'd think it would be a one-shot deal.
Since surviving it is (these days) a sign of divine favor, I'd agree. However, if it was used as an ordeal poison back in the day, it would be more like putting on a trial: you get one per criminal incident (however many crimes that incident may have entailed).
Say you've got a local guy of high rank (a psychopath, for ease of reference) whose family has a long and proud history of strong resistance to kresh toxin. He just murdered his uncle. He demands trial by hak'len tea.
People maybe grumble a bit that he has an unfair advantage, but tradition is tradition, so they put him to the test, and lo and behold he survives. He then goes on to rob the town trader. Trial by tea. And to stab his neighbor in the lung, who then expires after lingering for two agonizing weeks. Trial by tea. And to rape an eleven-year-old girl. Trial by tea.
By this time, you can see where somebody would start thinking they ought to increase the concentration. By the time he's fled town to escape the mob with pitchforks, become leader of a pack of marauding bandits, slaughtered entire clans apparently for the fun of it, carved a bloody swath across the countryside, kidnapped two members of the local royalty and returned them both in installments
after the ransom was paid, and made a good attempt at uniting the region's bandit tribes under his rule and using them to establish himself as a de facto tyrant ...
... by that time, you can see how, when he's finally captured under circumstances so public that you can't very well just kill him (gotta make a good show of it), and he
once again demands an ordeal trial by hak'len tea ...
... you can see how somebody decides, "All right, asshole, let's see you survive THIS."