Quiz

Well, Uranus is right. That is, even if there's only one prisoner, what's the problem? Give him a portion of kvass, then another (from a different barrel) a second later, and so on, a total of 240 portions. After 12 hours, we check: did he die or not? 12 hours and 1 second: did he die or not? 12 hours and 2 seconds – ? And so on. The second he dies is the second the poison took effect.
It all depends on the accuracy of the recording and the accuracy of the "poison takes effect after 12 hours" indicator.