It is certainly legal not to declare a draw after it is allowed under the fifty move rule--that has actually happeend in GM games (For example, Timman-Lutz 1995, where a rook and bishop vs. rook endgame ended two moves after it should have). The same is true of threefold repetition. The same is NOT true of insufficient material, obviously.

That said, the longest possible game in which both players agree to draw by the fifty-move rule or threefold repetition at the first given opportunity, but not by agreement, would be somewhere on the order of 5800-5900 moves.

Now I wonder if I'm not mistaken -- should I have used 50 instead of 49.5 as the multiplier before removing the half-tempos?  I think I should have.  That gives a ceiling of 5900 before removing the requisite half tempos (and, assuming I've got the right number of tempo-shifts, a final figure of 5897.5).  In either case I'm relatively confident that the approach is sound.

Incidentally, I wonder if this isn't exactly the calculation used to arrive at the figure of 5950 found on chess-poster.com in the original thread -- mistakenly adding in an additional 50 moves from when the players are down to just the two kings.

My head is beginning to hurt.

I come to a ceiling of 5841 -- here's how:

• 49.5*(8x6)x2 for each of the available pawn moves up to and including promotion = 4752
  
• 49.5*22 for each of the remaining peice captures (note that eight captures had to be made by pawns in order for them to have made it past the opposing colour's pawns hence the use of 22 instead of 30) = 1089
  
• The resulting ceiling is 5841
  

Now here is the hard part: subtract 0.5 each of the lost half-tempos as we pass the "intiative" (a term I use loosely here) back and forth.  Note that before black's pawns can pass throug white's pawn chain, white must make way.  For this, he must have the "initiative" long enough to do so.  Whether white can make all of the necessary "holes" with one possesion of this "initiative" is not clear to me, and this is where I'm not confident in my calculations.  My gut tells me that the intiative must be passed five times:

• Once to white to make way for half of black's pawns
• Back to black to let the first half of black's pawns through and to make way for white's remaining blocked pawns
• Back to white for white's pawns to pass through the chain and promote
• Back to black to promote his newly unblocked pawns and capture all of white's remaining peices
• and then one last time back to white to have his king capture all of black's remaining peices.

The net result, assuming I am correct on the assumptions that drive the subtraction of five half tempos, is that I arrive at a longest game of 5838.5 moves.   I could, however, very easily be grossly mistaken.

another source that seems to validate my logic that it is possible for the game to continue indefinitely (no, I am not an attorney, thank you):

"""A game is not automatically declared a draw under the fifty move rule – the draw must be claimed by a player on his turn to move. Therefore a game can continue beyond a point where a draw could be claimed under the rule. Theoretically, a game could continue indefinitely under the rules though in practice, when a draw under the fifty-move rule can be claimed, one of the players is usually happy to claim it (Hooper & Whyld 1992)."""

http://en.wikipedia.org/wiki/Fifty_move_rule

OK, here is the exact 50-move rule you quoted,  is there not a contradiction between subsection a and b?.....in section b, who declares the draw, and if that is true, why the need of part a, with one of the opponents declaring it (?)....section a makes it sound like a draw must be claimed by one of the payers, section b seems to say it exists as an entity despite one of the players claiming draw....

poorly written rules, IMO, and very curious....hmmmm, maybe Fischer had some merit in his criticisms of FIDE.....it is now well know some positions take more than 50 moves to mate, can a biased Tournament Director step in and declare a draw before mate, but after 50 moves have expired, if it happens to slip the notice of the player at the disadvantage????

“The game may be drawn if each player has made at least the last 50 consecutive moves without the movement of any pawn and without any capture. (See Article 9.3)
…
9.3   
The game is drawn, upon a correct claim by the player having the move, if
a. he writes his move on his scoresheet, and declares to the arbiter his intention to make this move which shall result in the last 50 moves having been made by each player without the movement of any pawn and without any capture, or
b. the last 50 consecutive moves have been made by each player without the movement of any pawn and without any capture.”

If you want to be absolutely rigorous to the rules, there should not be a "longest possible chess game", since if no player is willing to proclaim the draw, it does not occur (if I understand the rules correctly, feel free to correct if I'm wrong about this possible "loophole", then this point is moot, of course)....draw does not get forced unless certain conditions occur along with the 50-move rule, I believe (?).....so, two stubborn people with aversion to draw, and very long life could play until they drop dead.....even longer games possible for two computers programmed never to request a draw.....

(yes, I know this is beside the point, but a  possibility that should be considered?)

and i thought my 111 moves in a chess game was long.

And the number of possible FIDE legally board positions is....?

(more than the stars in our gallaxy?)