I turn my air conditioner on when I get uncomfortable in the house.
It's in the 80s F outside and feels much warmer inside due to the humidity in the air. I can't stand too hot of a temperature and start to get dizzy and feel weak and can't concentrate on anything.
I use a window air conditioner and only cool the room I'm currently in at the time.
My window air conditioner cost's me around $25.00 per month to run. Well worth it.