I don't vape, but I imagine that the user has a relatively good idea how much they inhale and how much liquid they use per day (or week). It's not like they switch devices, settings, and liquids all the time.
Also: most vapes have a trigger counter.
People do switch juices all the time so it is common to have different bottles of varying quantities sitting around.
If you're just vaping with whatever you can get your hands on and conceal as presumably teens are, I doubt they have a nice repeatable setup with a trigger counter.
But regardless, there are none of these confusing points in branded cigarettes. One cigarette is one cigarette end of story.