You need about 8-12 hours if you are using small jars and 12-15 hours if you are using bigger jars.
And this is an important step that many people miss… don’t stir it before you put it in the fridge!
You can give it a bit of a jiggle after 8-9 hours and ensure that the top is reasonably set in the jars but never dip a spoon into it or shake vigorously or you risk breaking the set.
Carefully cap the jars and move them to the fridge. Allow it to chill for a full 24 hours.
At this point you can eat it straight as is (my favorite) topped with jam and fresh fruit. Or you can strain it through a strainer that you have lined with several layers of cheesecloth (or if you really love greek yogurt invest in a chinois so you don’t have to use so much cheesecloth).