Spoiler (we need a ROT13 hover mode or something):

I don't see how he's wrong. I think we can both agree on the first part, and that that takes up 30 minutes. The second part is probably the contentious one, and it seems to me that that's correct, too. It's already burned 30 minutes, and burning from both ends means it's going to burn the remaining 30 minutes at double-speed.