It definitely varies depending on the source material. Using my settings, it's actually kind of comical looking at the bitrate counter on Kansas' Leftoverture, which spends most of its time below 150kbps, is usually around 130kbps, and on occasion even dips below 100kbps! It's a truly crappy sounding album, with almost no treble energy, barely any imaging, and lots of uninteresting "wall of sound" arrangement. This presents little challenge to the encoder; the average for the original album (non-bonus) tracks is 136kbps. Most of the rest of my collection has an average of >170kbps, and some of the more complex and shimmery-sounding albums (the Yes and Genesis stuff, mostly) push that closer to ~180kbps.
Like I said before, VBR AAC (NOT what iTunes gives you; you need to use the encoder backend I linked in my last post) is incredibly efficient. It's a little fiddly getting it set up, but you only need to do it once. After that, you're probably saving a good 25% of space over what you'd otherwise need to use to achieve the same overall fidelity.
Of course you know you're an audiophile when you a) know the difference between ABR and VBR, and b) know that iTunes doesn't expose the latter capability of the QT AAC encoder.
Edited by Argyris - 11/20/12 at 4:47am