It varies by individual tree, but here in Zone 6b most seedling trees will start by mid September and be done by mid October. Naturally, there will be an occasional tree that is earlier or later than the norm. The AU trees are Chinese. The AU IV is supposed to go into November.
They drop when they drop. I don't mean that in a smart way. I am fortunate to have permission to collect in a thirty tree grove of chestnut trees. There are like 5 trees that lead the pack. There are 6 -7 trees that drop significantly later than the other trees. In 2016 all of my trees were a week later in my locations I could collect in 2015 and in 2016. We were within one day of equally the most 90 degree days in northern middle TN.
I got concerned about my chestnuts trees. We got two timely rains and the burs swelled up well. They grew longer and dropped one week later.
Trees can vary from year to year. I had rather have a good rain in July then one good in August and one in early September. If I can three rains spread out in northern middle TN, my chestnut trees will do well.
On a specific tree, the south side of the tree generally drops chestnuts first if the sun is not obstructed. That side gets more sun and thus the burs drying there first.
I am five miles from the KY state line between Nashville, TN and Bowling Green, KY. Some of my trees I collect under finish up in late September. Others finish up in October. From the first dropping chestnuts to the last dropping chestnuts on a tree is about 2 1/2 weeks to 3 weeks.
If you get a hard blowing rainy thunderstorm you can see drop times drastically change. Ask the people that suffered the hurricane what happened to their acorns?
To over simplify, September some trees, October other trees. I have not found a November Chestnut tree - what I would give for one of those.
When you inspect a tree, look at the color of the burs and the conditions of their seams. I think we see the seams change visible first and begin to split. When you inspect a tree, walk around the full perimeter at a great distance from the tree so you see the bottom, the middle and the crown. What do you see? Any dead places? How many burs are clustered together? Any limbs touching the ground or stretching down to the ground?
I hope my answer has helped you some. I collected about 14,000 chestnuts off 46 trees. I shipped 8,817 chestnuts this year.
I am sure glad those 46 trees didn't all drop at the same time.