You probably have time to listen to more audio than you think. This, from Mark Shead at Productivity501:

For the past 5 to 7 years, I've spent about 2 hours per week listening to spoken audio. (This almost seems trivial compared to how much time the average American spends watching television.) That means over the past 5 years I've listened to about 500 hours of lectures, books, technical discussions, etc. That is four times the classroom lectures included in a typical bachelor's degree. Four times! And all I had to invest was time spent on long drives (when I usually get bored anyway) and while exercising (I spend a lot more time in the gym when I'm listening to something interesting).

Assuming his figures are correct, by my rough estimate, we have over 5 bachelor's degree's worth of audio here at desiringGod.org.

Of course, nobody wants a degree taught by just one professor, so be sure to find some other teachers than Piper, too. Let's say you just take one "class" from Piper per term and it's all lecture, no group work.--You'd exhaust our audio in about 13 years. Happy listening and let us know when you graduate!