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?
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.