When they're the size of small olives.
Keep the largest, most perfect one of each cluster.

Sooner is better. As I understand it, you can limit/control biennialism in apples by getting the aplets off immediately, even if that means removing blossoms. Your ultimate goal is to space apples to about one every 9", or thumbtip to pinkytip on a wide opened hand. But many people leave some cushion, as there will be June drop of the unpollinated apples. If you spray with carbaryl (Sevin) it will do a lot of thinning for you, maybe too much. And a certain number of apples will be damaged by bugs and birds and such.

I start to thin when the difference between the pollinated and the unpollinated fruit becomes evident, as the stems of the unpollinated apples turn yellow and the fruits drop.

I have thinned entire clusters of flowers on varieties that are stubbornly biennial. Generally, if you thin within 3 weeks of petal fall you get most of the benefits. By about 9 days after you should have a good idea about what has set.

I believe weather has a big affect on how well this will happen- if the tree can't store a lot of energy due to cool wet weather during bloom through early post petal fall it may reject much of the crop even if flowers were pollinated.

The people using chemical apple thinners have taught me a lot about weather and fruit drop. During those first weeks after bloom there is a large effect of carbohydrate levels in the tree vs fruit number and growth rate. Things increasing photosynthesis and carbohydrates hold more fruit on the tree. If energy levels in the tree fall, fruit falls off more and sooner.

Their thinners are much more effective during warm cloudy weather than during cool weather with sunny conditions. Warm weather increases fruit growth and carbohydrate needs. Clouds reduce carbohydrates and the fruit falls off.

A few days under total shade will knock all the fruit off the tree.

Fruitnut, that's good information. I bagged a couple dozen apples on my first-fruiting Fuji, and I can see that some of them dropped off overnite. We've had nothing but constant rain and overcast since the beginning of May.

And, of course, it's June. Since the Fuji was the first apple I thinned and bagged, I'm seeing that the dropped fruits are all as small as they were when I did it, failed to develop.