Most of my curling irons turn off automatically after an hour, I turn them or my CHI flat iron on when I'm blow drying the hair. They all heat up so quickly that they don't need to be on all day to be hot.
I have a few coworkers that leave theirs on ALL the time, even overnight.

I have one that doesn't shut off automatically, and sometimes it stays on for a few hours, but not on purpose!