When they start to wilt. You'll see when water lettuce and water hyacinths are being affected by the temperature, especially the lettuce. They are more susceptible than most plants.
That being said, if leaves are falling from the trees and you are covering the pond with a leaf net, you may want to remove all that tropical stuff before setting up the net. You don't want to be lifting the net to retrieve dying water lettuce or other tropicals.
That's what I do. The leaves dropping are the guide for me. I hate pulling tropicals when they look ok, but I don't want my pond filled with leaves.
One year I didn't net my pond due to a death in the family and my pond was a mess.