I have wondered about this.
If discussion lists count against your disk quota, I can't tell. I have a little discussion list that we moved from Yahoo. Bloody ads... The archive page says "...you can download the full raw archive ( 44 MB )." That seems rather large to me but what the heck.
If there is some kind of auto delete feature, I'd like to know just so I can d/l the monthly archives and toss them up to the Yahoo space we don't use (just in case someone needs them).