I'm trying to help a local college migrate from Exchange Server 5.5. One of the servers has a 105GB private store, and I want to perform an offline defragmentation to shrink the store before the migration. How long will this process take?

The answer depends on the server hardware. Typical dual-processor servers with RAID controllers can usually process from 3GB to 5GB per hour; so a 105GB database could take more than 30 hours to defrag. That's a long time to have those mailboxes offline. But as you've obviously noted, a database of that size will also be a chore to back up and restore. If the server's event log contains instances of event ID 1221—which would indicate a significant amount of white space—you probably should go ahead and do the defrag. Exchange 5.5 doesn't manage database white space as efficiently as Exchange 2000, so defragmenting the database before the move is probably a good idea.

An alternative is to install an Exchange Server 2003 or Exchange 2000 Server system, then use Exchange System Manager's (ESM's) Move Mailbox feature to move all the mailboxes from the Exchange 5.5 server to the new server. Doing so doesn't require you to take down the entire Exchange 5.5 server and will shrink the database on the target server because a database format conversion takes place between Exchange 5.5 and Exchange 2003 or Exchange 2000 (although pages are converted only when they're accessed).