Trick question
Don't use Furmark at all. Furmark kills GPUs with ease, so there are better ways to test for stability.
Check out Unigine: Heaven and Unigine: Valley. They stress the graphics card just like a top-end game would, and there's no harm to the graphics card unless you've OCd it wrong.
Longer the test period the better. I'd say that if you keep amending your clocks/volts etc. test it for a good 20 minutes at a time.
When you're satisfied and want to test a '24/7' overclock you'll want to go for longer. 1 hour is not enough. I'd say 6 hours is a good time to have it run on loop