daemon:

If you are certain you are past freezes, it's good to start early. Even as early as blossom thinning help peaches that mature early. Later maturing fruit it's not as much effect. I never wait for natural thinning to take it's course first. But then I've been known to go overboard on some things.

One of the first things you can do is pick off the twins.

I agree with Fruitnut and Randy At least go through right away and pull off all of the kind of corroded type fruit.

You don't waste a lot of time if you make a few passes at the job. You also lessen risk. Suppose a pretty big weather event takes off some of your fruit in a couple of weeks. You would be glad that you did not make the definitive thinning too early. To me its also psychologically easier to do it in phases, as I'm predisposed to allow the tree to overbear otherwise. Keep on top of it, though. If you find yourself thinning a bunch of fruit much bigger than a dime, you waited too late.

The sooner you thin the bigger the peaches you harvest. However if you get another semi-hard freeze you may find yourself with half or more of your crop affected- although that's only happened to me once in 25 years in southeastern NY.

I would thin them to one every 3" and come back later in a month an thin the rest.

I'm done thinning peaches here. Well, not really. It seems like no matter how thorough you are, you can still walk by any tree and see some fruits missed.

As mentioned, earlier thinning produces bigger fruit. The article below explains why.

It's my impression people new at growing peaches don't thin enough. In my own experience, I tend to under thin. I have over-thinned before but that happens only on younger trees I've thinned very early that have a thin fruit set to start with. They'll sometimes drop more fruit leaving an over thinned crop.

It's common advice to thin early varieties more heavily than later ones, but I can't seem to make this advice work. Certainly early varieties need to be thinned heavily to achieve any size, but later varieties are generally so big they put too much load on the branches if not thinned aggressively.

I was telling my wife the other day this business is psychotic. First you hope and pray the frost doesn't take the crop. Then, as soon as that risk is over, you go out and remove 90% of the crop. Then the rest of the season, you fight like mad to protect the 10% you have left.

Some people just thin as they go, constantly removing damaged peaches, which is OK. It does put more stress on the tree (More energy is going into larger fruits that will be removed.) Also, the peaches won't be as big or as sweet, but they will be better than what you could find at the grocery store.

Here is a link that might be useful: How to grow big peaches

Well I picked half of them. Still plenty..