Sorry for only responding now! Crazy couple of months behind the back and the future does not look like the train is slowing down haha.
Anyways, I leave it overnight in the fridge! That has always worked for me. The layer of yogurt closest to the cloth (on the bottom) will be extremely thick, cheese-like thick, and the top layer will be runnier. But when you mix them after the strain, they will be perfectly thick but not too thick, if that makes sense.
RE: Purslane and Spekboom Tzatziki: A Simple Homegrown Recipe