1-9 hours. Run it when you sleep or go out some where. I personally test for stability for ~5-7 hours or go to sleep and have prime 95 running and then see what happens. Say I go to bed and then wake up the next morning to a system that either is still in windows or is completely unresponsive.
Well after 1hr 45 mins my system just shut down without warning, but no errors appeared in BIOS.