I know there is one, found it on a site some time ago. Let me look again...
Here we go, though I do not understand it all, it makes a bit of sense to me:
"How long is the longest possible chess game?
The basic idea is a player may claim a draw if fifty moves elapse without a capture or a pawn advance. Ignoring the special cases where more than 50 moves are allowed by the rules, the answer is after Black's 5948th move, White is able to claim a draw. The simple calculation is (<Pawn_moves + <Captures>- <Duplicates>+ <Drawing_interval_grace_period) * <Drawing_interval, or (16*6 + 30 - 8 + 1) * 50 = 5950; we're able to trim two moves from this total by observing that sequences of Captures/Pawn_moves must have (at least) 4 alternations between the two players."
PS: Found it here: http://www.drpribut.com/mwiki/index.php?title=Chess_Faq_Part_Four