Bare minimum (means it should never fall under it) 25.
acceptable 30
great 40
Perfect 50 or more.
THe more varaible the frame rate is, the more it seems to "lag". So if coding of a game is very bad, it will look unplayable even at 60 fps.
This happens becouse our eyes and brain get used to a certain frame rate, and if it keeps changing, our brain needs to keep up with it (impossible at how fast it changes). From there comes the feeling that a game is unaplayble even at higer frame rates than normally playable.