I leave my rig running 24/7. 3x 7950 GPU's, Temps 50-65, 1.6MH/s stable. I can ramp it up to 2MH/s at the weekends but I don't like doing that. If it's over 2Mh/s it might stop at 3am, then I'll find out at 7am (etc etc). Not the best, so I've opted for stability.
I could push the cards a lot more but I prefer the stability. And I have a day job. I don't want to have to have to worry about it (it's in my attic/loft). I go about my business and check it once in a while.