Depends. How good are your eyes?
If you're a heavy gamer playing FPS, you might not like it if your framerate drops below 40 or 50. Especially if you like moving your mouse quickly.
If you aren't, 30's usually more than playable with most FPS. Some games cap framerates, I suppose to save power and heat. FPS often cap at about 60.
For RTS, I would say you want to keep it above 25, but it doesn't matter so much. CNC3 is capped at 30fps.
So, 30 fps is a good approximation.