Assuming the original definition of a second, which is a fraction of the Earth's rotation:
Earth's sidereal day (one complete 360 degree rotation) is 86160 seconds.
The Moon's sidereal day is 2352168 Earth seconds.
So one lunar second would be 27.3 Earth seconds.
But the modern definition of a second, according to the International Committee on Weights and Measures, is "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom." In that case, one lunar second is equal to one Earth second.
Unless the Earth is flat ...