Been a long while since I looked into it, but 40C sounds bogus. Seen plenty of people talking about boiling them to make tea without any noticeable loss of potency. You wanna get ALL moisture out of them, so if they flex much at all before breaking they may need to go longer. 18 hours seems to have been plenty for mine to completely dry. If you want to be extra careful, you could take a tray that you think is dry, weigh them, put them back in for another hour then weigh them again. At some point they'll stop losing weight and you know they're bone dry.