time-based? Like, assuming the time of death was pretty precise, you could assign each prisoner 100 bottles to sample, and they'd sample one every 5 minutes. That'd take 495 minutes, or between 8 and 9 hours, leaving plenty of time to wait up to another 10 hours, see who dies and when, and get rid of the bottle he drank 10 hours prior (or give it to all the other prisoners, since we're rather evil).