As with most of these things any truly scientific approach to the problem quickly highlights semantic tricks to the question.

If the burn rate truly is variable -- as Andy points out the legitimate but extreme case -- then that decouples burn time from fuse length, which invalidates the whole idea of being able to define 60 minute fuses but not 30 minute fuses

So it's a clever question for Mensa wannabes but not scientific theory.

--Nathan