I think more to the point for many people would be: what percentage of the time you've spent in school has actually contributed to your repertoire of knowledge?
I think a lot of people might answer something like 20% for that, too.
Now that I'm back in college, this time assistant-teaching as well as taking grad-level classes and sneaking in time for research, I am more amazed than ever by how wasteful the academic world is. They chop up your day into many context-switches (one of the worst things a knowledge-worker can do), they focus on lectures and exams instead of mentoring and experiences that bring about learning, they require useless courses, they emphasize grades relentlessly (known to decrease learning), and they present material out of context (so it's hard to retain or apply).
I think good teaching can be done in the academic world (I have seen some), and of course it's the place to go to do scientific research. But overall, the academic world is just not very good at what it does (except arguing about minutiae and winning research grants and endowments). The basis of its strong market position is subsidies and prestige. It's ripe for competition from savvy entrepreneurs.
I think a lot of people might answer something like 20% for that, too.