The Exchange Server Role Requirements Calculator has a great if seldom used feature to allow estimating of bandwidth needs between datacenters for replication traffic. The key to making this work is filling in the little chart of what percentage of a day's log files are created during each hour of the day. Since this is based largely on messages sent/received you can use your existing Exchange Servers log creation rate to figure this out.
However, it is kind of a pain to examine log directories and count the number of files created during each hour of the day.
This script will look in one or more log directories and do the math for you. You can then copy the output straight from the powershell window into the calculator.