Another piece falls into place. · 9:53pm Apr 28th, 2018
Wow! As of 'Horse Play', we now have a definite time for when Celestia and Luna first raised the sun and moon. 1111 years ago. This gives us some idea of when other things happened, though some accounts conflict.
The Journal of the Two Sisters tells us that Celestia and Luna grew up in the hills of Mount Canterlot, cared for by other Alicorns, who had apparently left. At the point when they were approached by the Founders (Smart Cookie, Pansy and Clover the Clever) and Starswirl the Bearded (who is explicitly stated to have a brown beard at that point), they were no longer fillies, being adolescent equivalent, and not yet possessed of cutie marks.
They accepted rulership of the three tribes, and built their palace in the Everfree, the Palace of the Two Sisters. After a relatively short period of ruling, no more than a few years from the context of the book, they raised the sun and moon. We can now put together a timeline with some approximate dates. I'm using Luna's fall as the zero point, as that's the only definite date we have). I'm also assuming that since we've had three Hearth's Warmings, the show has run over three Equestrian years.
Era of the Three Tribes (??? to ~120 BNM)
Fumblewinter/The Founding (~120 BNM)
Start of Pre-classical Era - Diaspora across Equestria, with various groups of ponies forming communities with divergent cultures that the Pillars arise from.
Discovery of Alicorns around Mount Canterlot
Formation of Crystal Empire by splinter group, led by Princess Amore?
Coronation of Luna and Celestia (~118 BNM)
Construction of the Palace of the Two Sisters/Consolidation of rule/Tutoring by Starswirl
Celestia and Luna raise the Sun and Moon for the first time (~110 BNM)
Tireak and Scorpan appear/Scorpan befriends young Starswirl/Tirek banished
Ponies spread and grow in number across Equestria.
Birth of the Pillars
Sirens raid Equestria
Stygian brings Pillars together (~60 BNM)
Creation of seed of Tree of Harmony - Just before the Pillars go to fight Stygian.
Banishment of Pony of Shadows (~45 BNM)
Tree of Harmony grows under Castle of Two Sisters
Sombra found and brought to Crystal Empire as foal
Discord appears/is sealed with Elements of Harmony (~20 BNM)
Sombra becomes Umbral vessel, kills Princess Amore and takes over Crystal Empire
Two sisters banish Sombra with Elements
Luna grows discontent with secondary status
Luna is infected with/gives in to Nightmare Miasma/becomes Nightmare Moon (0 BNM)
Celestia banishes her sister
Mane Six restore Elements/Free Luna (1000 ANM)
Creation of Scholl of Frindship (1003 ANM)
So, all in all that single century and a bit when the Princesses first ruled was pretty packed. I'd have guessed it would be several hundred years between their coronation and Luna's fall, but the show is ultimate canon.
Other sources:
Journal of the Two Sisters (Though that has a Tree of Harmony in place at the time of the Castle's creation.)
Fiendship is Magic 1 - Sombra's story
IDW Friendship is Magic Issue 2
I'm of the mind that the show has run for 7-8 Equestrian years. 1 year for each season of the show. 3 years feels far too short for everything in it to happen.
4849482
The cutie mark crusaders not growing at all for 7 years feels pretty ridiculous
Well, if
disastersepisodes happen weekly, then it's been just over 3 years. Six full seasons gives two seasons of 26 episodes per year plus the 13 episode half season minus 14 episodes for the second episode of each double length season opener/finale story.May be the ultimate canon, still probably going to ignore it though I might shrink Discordant Era and Pre-NNM Era...
4849482
Three Hearth's Warming Eaves: one anytime after Twilight gets wings, but before Tirek attacks, one after Starlight becomes a Temporal Assassin, and one before Twilight Ascends. Two Summer Sun Celebrations: one on the Eve of the Second Eternal Night and one on the Element's Return to the Tree. Two Nightmare Nights: one before Twilight's Ascension, with one after. Two Grand Galloping Galas: One before Discord's Escape and the other is after His Voluntary Reformation. One Hearts and Hooves Day, one Running of the Leaves, one Equestria Games, and one Crystal Faire.
One (School) year since Twilight's Ascension, at least, has passed; it was practically confirmed canon by show staff that one year elapsed between Summer Sun Celebrations.
tl;dr
At most, three years have passed by the end of this season, with it being highly likely at two years current.