When we had air con back in the 80's when I was at home as a kid I think we turned it on in the mid 30's.

It's a pretty touchy subject with me LOL I LOATHE the heat but I also am a "tight-arse" budget-wise and environmentally conscious person. So ATM we don't have it... we make do with a fan. Fortunately we live on a hill which seems to catch the slighest breeze and live in a house that can be kept comfortable (if the kids can be stopped from going in-out-in-out all day).

I really suffer in the heat but I think I'm happy enough just to "adapt" to the weather. I don't want my kids to become to reliant on a/c either... so I help them keep cool in other ways. We have ducted heating and my teen DD is bad enough as she 'abuses' that let alone a/c.

But to answer your Q if we did have a/c it would only go on if it was forecast to be a hotter than low/mid 30's day. I would still close up the house, draw the curtains etc to help the a/c rather than work against it by letting the sun shine in etc.