Sometimes I see windows with a cat net installed on it. How are they used? When should I consider using a cat net?
Any fall over 10 feet/4 meters could be dangerous to a cat. (Websearch "feline high-rise syndrome" for statistics). Cats may be OK above that, or may be at risk below that, but that's the point at which I would start worrying. (A past cat almost stuck her landing from 15', but banged her jaw on the ground and broke it.)
As with kids, the risk depends in part on how careful the cat is or isn't, but sudden wind gusts or a misstep are always a risk.
Insect screens aren't adequate to protect kids, but can do a decent job of protecting most cats, and are probably more useful and more attractive than netting...?