On my servers i have ~20% recurring visits and ~80% one-time visits. So for me it would save 30%*80% = 24%.
This is something that I observed a long time ago. CSS is often a large part of my web sites' bandwidth usage. I wrote about that One site has .css files taking 10.5 % of the hits, yet 19.6 % of the bandwidth. Another site with a different theme has .css as 11 % of hits, and 13.6 % of bandwidth. That site has more css that the first one, since it uses event module which emits its own css. Both sites have the css compressed, although I remember the latter one having an issue with the compressed version, so I reverted back to the normal version. I forgot about drupal.css. I will compress that next, and copy it. Thanks Piotr for the reminder.