If it's unstable you'll usually know within a minture or so of running the benchmark or program. As soon as it tried to ramp up to full load the failure/crash to the desktop with a driver error will occur. So, if you can get it to run for anything beyond 5-10 at full load with reasonable temps and no crashes then you're generally good to go.
It won't hurt the card in any way. The cards are designed to operate up to 100'C at which point most of them will either down clock or crash to the desktop preventing any sort of damage from occuring. That said, you should never see anything beyond 80-90'C with most reference cards and half way decent case airflow.