That will vary by:
a) case cooling
b) the game
c) your GPU (varies even among same model of card)
Basically, try overclocking by 5% then run for a while to see if artifacts appear or you get crashing. Just because one game works doesn't mean another game won't.
If you already get 60FPS and use VSYNC then it's pointless to overclock as your bottleneck is the VSYNC cap not the card.
If you manage 10%, I'd stay there.