Measuring the shelf-life of student interest in their subjects, using Google...

Just because someone does not read about slavery all the time it does not mean that the time he read about slavery he learned nothing. Same with postmodernism or evolutionary psychology. Semester after semester you have different things to learn about and if you get stuck into slavery or postmodernism you might fall behind on the other parts of history or literary theory.

The guys thinking is same as suggesting that during holidays people do not work therefore they are not learning or doing anything while they are working. I still remember some stories from literature of primary and secondary schools, I still remember how to multiply and do fractions and percentages and the pyntagora theorem etc, although I have not spent most of my summers doing any of them. Learning is not about keeping the interest in it, is about telling you what is and what can be done with it. What you do with the knowledge might not be handy when you are still in school, but once you're finished and into the real life much of it will become perhaps crucial as fractions are just the basic thinking blocks to further more complex abstract thinking needed for maths and that simple story in literature is the building block for later more complex stories.

In short learning is about shaping ones mind not necessarily maintaining interest in any one thing.