When they are ripe!

MG has the only true answer, Mark. Your photo apple looks pretty ripe, but the only way to really tell is to pick it and eat it. We do the same with 'Anna' apples and citrus here: when the season comes near, we pick one and eat it every few weeks, and when it tastes good, we harvest the rest on the tree

if you twist the apple instead of pulling it and it comes of then they are ready to eat cheers , H

If the fruit leaves the tree with no resistance, it is usually ripe, or has/had a lodger. Put your hand under the selected fruit and raise gently......if it parts company....enjoy.