I recently ran into some issues with our off-box message store, and all of the research I came across pointed to performing an offline defrag of the message store. I also came across information regarding how long it would take, about 1 hour per 5Gb.
Say I have a message store that is 76Gb, but only 11Gb of that actually contains messages. The rest is empty whitespace. Can I assume that I will be looking at a 15 hour defrag based on the total size, or maybe just a 2 hour defrag based on the size minus the whitespace.