How this works for me

Breaking my study down like this into manageable chunks is the way that I like to study normally. Like your teen, I would feel extremely low at the prospect of having an infinite number of study hours ahead of me. But if I set myself a manageable 1 2 hour task, then thats all I have to focus on for that time I put can in the energy and effort the task requires without feeling overwhelmed.
Once I complete a task I usually have a break make a cup of tea, have a squiz at whats going on in the world then Im ready to launch into the next task. How much study your can teen do in a given time will depend on them. But if they structure things so that they study in these task based chunks, they should at least be studying productively.