Considering that most audiobooks (after the discount) fall in at around $0.75/hr, you'd have to be listening to 4ish hours a day to break a hundred bucks a month (unless you listen at a higher playback speed; I don't). 2-3 hours a day is probably my personal average.

I'm not one to bash pirating - I do it for audiobooks with exclusivity deals from time to time. But I hope that if you are pirating all your audiobooks that you donate to the author directly to make up for it.