When the trichomes are cloudy. Here’s a video that discusses potency at the various flowering stages.
Peak THC in resin glands is when they look milky/cloudy
Too early and they are clear. They have some THC but may be weak and may give you a headache
As they age they go from milky/cloudy to amber. This is THC that has degraded into CBD and CBN which helps mellow out the THC effects. Many growers want to see about 5% to 10% amber to balance out the high. Others want 20% or more to put them in couchlock and then sleep. Some want no amber at all. No right or wrong, just personal choice.
You want to look at the flowers and not the sugar leaves. Flowers will have pistils protruding from them. Almost all the pistils should be browned before you begin checking the trichomes.
That amber one looks