See, and here I've gotten the impression from the manga that the contract is basically for however long Ciel lives. Spears' remarks about how odd it is that a demon should hold off and actually seek to preserve his contractee's life, and Sebastian's jibe that it makes the taste sweeter in the end, seemed to imply something like that.

*rather amused* Yeah, if that's how its played, I could see that. Ciel is ruthless; I could totally see him refusing to back down to the very last.