Hi Liz,
There was a thread elsewhere on emaildiscussions.com where email service providers were discussing on average what proportion of users' max storage quota the users were actually using.
I think most numbers were in the 30-40% range (so for e.g. if the average user was entitled to 1 GB storage they used 300-400 MB storage).
What are the similar stats for Runbox? Would be interesting to see