So, we know that the Mirror of Erised was the ultimate guardian to the Stone in the first Harry Potter book, but we also know that it wasn't there until about the New Year. The Stone was there for perhaps 4 months before that. Was it just as protected as it could be during those months, or perhaps the mirror was moved back briefly so Harry could look into it? Just curious if there's ever been anything official on this matter.
As far as I very vaguely recall, it was only explicitly stated that it was Fluffy (Hagrid confirmed to them it was guarding something when they discussed Snape after the Quidditch match).
However, the rest of the magical defenses in the 7 chambers - aside from the mirror - were probably also in place at that time as well, although there is no information about it in any sources. The most comprehensive reference seems to be http://harrypotter.wikia.com/wiki/Philosopher%27s_Stone_Chambers and it's silent on when each post-fluffy defense is added. However, it may be guessed that they were all added at the same time as Fluffy.
As far as the mirror, there's no evidence, but Harry himself says he suspects that it was placed in room for him to find (and therefore was employed as a defense prior to that) in the end of HPSS:
I don't think it was an accident he [Dumbledore] let me find out how the mirror worked. It's almost like he had thought I had the right to face Voldemort if I could...
We, the reader, assume that the obstacles that were revealed to us were the be all that ends all.
What if there had been rotating obstacles throughout the time the stone was at Hogwarts under protection?
I think after the New Year, when we know for certain the Mirror of Erised was moved, Harry's experience with it gave Dumbledore that moment of inspiration to add it to the obstacle course; he says so at the end of the book as Harry is recovering in the hospital wing.
I like to think that guarding the stone was constantly being updated and changed, since Dumbledore was aware that something was amiss; the break-in at Gringotts being the tip-off.
The Stone was at first in the vault at Gringott's. It's implied that when it was moved to the third floor, most but not quite all of the protections were already in place. The Mirror of Erised was likely acquired by Dumbledore for the purpose of protecting the Stone, but as the threat of Voldemort getting through all the existing protections was low, Dumbledore simply placed it in storage for a while until he had the opportunity to hide the stone using the Mirror. Harry found it, likely not surprising Dumbledore in the slightest, and learned what it did, which then forced Dumbledore's hand to get it set up hiding the Stone.
Remember that Quirrell, Voldemort's puppet, didn't know how to get past Fluffy until he'd gotten Hagrid drunk and talking in the pub, and he probably had issues with at least one other before the Mirror; possibly Snape's puzzle (which the movie didn't show; the puzzle in the book was not explained fully enough, with the sizes and shapes of the bottles described in their order, for the reader to reason it out). So even without the Mirror in place, the Stone was still quite safe even if it were just sitting on a pedestal in the final room, as long as Quirrell didn't know how to get past all the obstacles. He didn't figure it all out until very late.