Lets simply consider all possible patterns for the game. Suppose we have k pieces left on the board. Each piece occupied one space, so the number of possible arrangements is 64!/(64-k)!
The game start from k=32 and after a while the number of pieces reduces one by one. No matter what players try, after 64!/(64-k)! steps either the game will repeat itself or the number of pieces must reduce by one. Thus, if we assume that players can not play the game in such a way that it will repeat itself, the game will end in at most
N:=sum_{k=1}^{32}64!/(64-k)!
steps. Of course, this number might not be sharp. The point is we can consider a game as a function from {0,1,...,n}, n