That is a subjective question and depends on both the person and the hardware. "Microstutter" is nearly unavoidable but also is not noticeable by everyone, so that's where hardware comes into effect.
Some people also find 30fps to be totally ok, some can't stand less than 50. Whatever the case, most people notice a difference in FPS more than the actual number. So if you go from 120 to 60 you might notice that 60fps drop, but if you go from 60 to 50 you probably won't be able to tell.
What is the point in asking these questions? Like I said, it's subjective. Depends on you and what you're willing to deal with.