It varies depending on the individual card. Get a stress-testing program like Furmark or Kombustor and try it out. Work your way up in the core clock first, then in the memory once you've found the highest core clock you can handle. Test each setting for at least 10 minutes, and at least half an hour on the last test, when you're going to call it quits, to make sure you're solid. Don't overvolt.