We turn ours on when the temperature gets to 79 in the house, and it's set for 79F.
Since we're at 5000 ft elevation, with super-low humidity, and have a super-insulated house, on really hot days that means it usually comes on about noon to 1 p.m.
By 8 p.m. it's cooled off to 75 or so outside so the a/c goes off and the windows get opened. I love the high desert!