While the antifire potion is a miraculous thing - one of the most useful items on Ashenfall - it does have a flaw. It can be hard to know when its effect wears off. Knowing that you are protected, or not protected, while standing in front of a dragon is something of a problem.
One solution is to bring someone with you. Get them to count while you fight a dragon. Then they can give you a happy thumbs-up while protected and a sad thumbs-down when you're not.
Of course, they probably need antifire if they're going to assume that role. Which means a third-party to give a thumbs-up or thumbs-down for that second person. Yeah, we see the flaw in the plan.