Just found a new semi-canon answer to how long a "moon" is. I just read the most recent "Friends Forever" comic, and read this:
"You must have become a legend in Equestria during the thousand moons the crystal empire disappeared..." - Twilight Sparkle
We already know that the Crystal Empire disappeared for a thousand years. So, according to this, maybe a moon=a whole year?
Only, this, of course, begs another question: if, in Equestria, a moon is a year, and an Apple Family Reunion happens every hundred moons... Not only is Granny Smith hundreds of years old (along with possibly many other characters), but if the first episode was a reunion...the series' duration has been at least a hundred years! Although, this would give some more sense to Twilight's comment in "Winter Wrap Up" about how long the Earth Ponies have been changing the seasons by hoof instead of with magic
So, either ponies age REALLY slowly, or, one of the writers of the comics flubbed. I lean towards the latter personally, but will not let the fact that the former might be canon slip away.
Does anypony know at the moment if "Canterlot Nights" Decks are inter-playable with Premiere decks? For example, say I have a Twilight deck, and my opponent has a Celestia deck. Can you still play, or are there conflicts?