I was looking at a post by Billium where he was asking what is the longest opening ever accomplished here on cess.com and someone made mention of the idea that his example was "clearly" into the middle game.  So my question is what actually signals the end of the opening?  I understand Billium's point that he's considering all of the moves with all of the variations as an opening is this correct thinking?

nothing:)

Its purely dependant of the opening in question. Generally when you come to a point where the position has a lot of playable ideas/plans without a well established main sequence of moves, you've entered the middlegame. This could happen as early as move ten or as late as thirty, but it all boils down to the opening chosen.

It's much like the question of whether knights or bishops are the better minor piece. The only correct answer is "it depends on the position".

For popular lines, as soon as you are out of book, you are in the middle game.

True, many "opening" lines have been analyzed well into the middlegame, which is possible when the reasonable moves on each side are limited or forced.

To me, the "middlegame" has begun once both sides have set their central pawn structure (or plan), completed their development of minor pieces and Queen, castled if they intend to, and deployed their Rooks whereever they intend for the initial stages.  The position may well still be "in book," but it's a middlegame from then on.

well you can take something like this well know theory in the panov botvinnik

Some (slightly contentious) definitions here.

For the endgame, I've read somewhere that it is when both you and your opponent have two pieces or less left, regardless of how many pawns there are.

Just listen for the chime.

Ah, parting is such sweet sorrow.  Don't we hear the chimes at midnight?