Last time I tried air layering I cut it off too soon and it died, so I am a little hesitant right now.

I have a hornbeam in the back yard air layered using a plastic kit, and it has lots of root showing in the clear plastic. Now it is mid September and most of the trees are coming out of their summer dormant period. Leaf drop will come in a month and a half, end of October to mid November. Do I cut if off now and put it in bonsai media? Wait till leaves fall off? Wait till spring? Put in bonsai medium? Try to keep the peat moss around the roots?

Similar question about air layering a maple tree - I did this in early June. I haven't looked for roots because that would involve climbing a latter on rough ground and I hesitate. Most of them have kept the leaves green, so something positive has happened. When would I expect to cut them off?

I can climb the ladder looking for roots. I need excitement in my life, but that might be too much. (When I was doing the air layering the ladder fell and I was left hanging. I managed to drop, miss the ladder, and stay on my feet. Live on the wild side.)

I separate as soon as roots are visible.

Key is to absolutely in no way disturb the new roots. Just cut. Put in a container. Fill with substrate. Water. No pushing, tamping, cleaning etc. The roots can break very easily.

Will do.

And (I think obviously but who knows), put it in partial sun, water when substrate is dry.

Temps are dropping a little. High tomorrow is 32C then around 25, lows around 20 for a while, so there won't be much heat stress.