Sun Java System Calendar Server 6.3 Administration Guide Amount of Storage Needed

The formula is:

Amount of Storage Per User = 100 emails per week, multiplied by 52 weeks a year, multiplied by 5K per email, multiplied by the number of years worth of data to keep online, multiplied by the number of copies (5 backups + 1 working copy) kept online = 100*52*5K*2*(5+1) = 65 MB storage per user.

That is 2.6 MB per user per year per copy held online.

Note –

The final number depends on how many hot backups or archival backups you keep online. For this example, 5 backup copies was the number used.