It's different for every GPU, try going up by 10Mhz on the core and do a run of firestrike or similar, if the temps are fine and it doesn't artifact/crash, it should be good, add another 10mhz and repeat. Don't try and jump to other peoples overclock speeds, it doesn't go well sometimes, I reached 1500Mhz with my 970 but it doesn't mean you will. When you start to get into the higher overclocks, you should stress test it for longer.