Try taking a look at this article, it's an estimate but should at least help.
http://technet.microsoft.com/en-us/library/bb125003%28EXCHG.65%29.aspx
Mark Morowczynski|MCT| MCSE 2003:Messaging, Security|MCITP:ES, SA,EA|MCTS:Windows Mobile Admin|Security+|http://almostdailytech.com