With the fifth episode of Season 5 - Tanks for the Memories, we get one of the very rare indicators of the passage of time inside the Pony universe. I actually think that we've got enough data to make a stab at that controversal question: How long has Twilight Sparkle been in Ponyville and what has the passage of time been like in-universe?

My Rough Back-of-the-Envelope Calculation

So... How long has it been?

The Summer Sun Festival to Fall Weather Friends = 15 Months or so (Mid summer, through a winter (Winter Wrap Up) to autumn again.

Fall Weather Friends to Hearth's Warming Eve = 3 months (assuming Hearth's Warming Eve falls on the shortest day of the year and the Running of the Leaves on the Autumnal Equinox)

Hearth's Warming Eve to Princess Twilight Sparkle - Part 2 = 6 months (midwinter to midsummer)

Princess Twilight Sparkle - Part 2 to Tanks for the Memories = another 3 months or so (midsummer to mid-autumn).

Yes, I'm saying that Twilight Sparkle has only been in Ponyville for about two years so far and the majority of episodes in the show to date (since S02e11) has occured in just one Equestrian calendar year!

If there are any better continuity nerds than I out there, I welcome a better analysis than mine.