You will need to try it yourself as all cards are different when it comes to how far they can oc. Try going up 10-20mhz and running a 3DMark benchmark, if it doesn't crash/artifact/overheat then it's probably stable and you can add 10-20mhz more and repeat. If you see the score drop at all, or it artifacts/crashes, back it down by 10/20mhz and test it for longer using a game or benchmark loop.