They're tapped when used and untap for reuse at tick 10, unless specified otherwise.
It's a mechanic, don't try and force "fantasy logic" on a game. It never works.
Actually, I think there is any easy way to explain this. The potion has several doses. You can only consume a certain amount (i.e., a dose) at once. Taking more than the dose doesn't do you any good. Once the master clock hits 10, then the dose has run its course and you're ready for another.