I think we need more bite sized media.. We are absorbing more and more.. I listen to more Boo's on the go through my ipod than i do on the site.
I think this is why i like audioboo so much.. i subscribe to a few in my itunes feed and know that when i listen they could be from 10 second posts up to (normally) 5 mins.. This means i get to enjoy more media from more people and when i fillow over 3000, i feel this is important.
So in a nut.. The shorter the better unless you are only doing it for yourself or you know you have gold.
Quite often i record audio to have a rant to vent rather than to share. I should really have a different account for those kind of days.. :) One I don't care if anyone listens to..