A moon is a unit of time used in My Little Pony Equestria Girls, and in the IDW comics. The length of time it represents is not determined, and when Friendship is Magic and Equestria Girls director Jayson Thiessen was pressed for its measure, he said it has "no human equivalent."
In early 2014, Ishi Rudell was asked "Since the portal to EQG land only opens every 30 moons, how long would you say a moon cycle in Equestria is?" and answered "Hmmm that's a question better left for Jayson Thiessen. I'd say it's roughly around the same as the human world, but then again, A moon rotation would vary depending on how big or small the planet it was rotating around?",  and later in late April 2014, Jayson Thiessen was asked "how long is a 'moon' in MLP? Like, the Portal in EG opens every 30 moons" and answered "it's a unit of time with no human equivalent." 
Depiction in the films
In both My Little Pony Equestria Girls and its screenplay novelization My Little Pony Equestria Girls: Through the Mirror, Princess Luna states that the Crystal Mirror gateway to the human world opens once every thirty moons and closes on the third day when the moon reaches its peak in the night sky.
In the audio commentary for the DVD and Blu-ray release of My Little Pony Equestria Girls: Rainbow Rocks, when asked "how many moons after" the first film have passed, Meghan McCarthy states, "Six. It is six moons."