My personal minimum is a stable 40/s, which is roughly where I start noticing the lower framerate without paying attention to it.
With 30/s I need to get used to it, and I usually underclock (or, rather, power-limit) my GPU to hit an average 50 unless the game in question is either highly unstable (e.g. Helldivers 2) or the game is so light I don’t have to care (e.g. Selaco).