Moons

A moon is a unit of time that is possibly equivalent to an Equestrian year, an entire moon cycle, or one-thirtieth of the cycle of the moon but is said to have "no human equivalent." Moons are used to measure time in multiple My Little Pony Friendship is Magic episodes, in My Little Pony Equestria Girls, and in the IDW comics.

Development
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 EQG opens every 30 moons" and answered "it's a unit of time with no human equivalent."

Depiction(s)
In the IDW comics' page 21, the disappearance of the Crystal Empire for a thousand years until The Crystal Empire - Part 1 is stated by Twilight Sparkle to have been a thousand moons.

In Apple Family Reunion, Granny Smith states that the Apple family has been hosting Apple family reunions at Sweet Apple Acres every one hundred moons since they first planted roots in Ponyville; a present-day Apple family reunion—as identified by Applejack in Applebuck Season—is shown in Friendship is Magic, part 1, and multiple past Apple family reunions and another present-day Apple family reunion are shown in Apple Family Reunion.

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 IDW comics' Reflections Part 1 page 4, Fluttershy states that this magical mirror only works once during each cycle of the moon.

In Princess Twilight Sparkle - Part 2, Discord states that his plunderseeds should have stolen the magic from the Tree of Harmony and captured Princess Celestia and Princess Luna thousands of moons ago but had been being kept from growing up big and strong by the tree's magic.

In Pinkie Apple Pie, Granny Smith states that a Ponyville snail can hibernate for up to forty-eight moons.

In Trade Ya!, Fluttershy states that training an orthros takes a long enough time and that her stay with "Teddie Safari" in Manehattan will last for "I don't even know how many moons."