When I ran eseutil defrag against Exchange server 2003 email data bases it only took a couple of hours to do a 60GB database. But it depends on the specs of your system, number of procs, how much memory etc. I don’t recall if there was a limit to the number of corrupt items you can have before moving a mailbox. When I moved all of mine I set it to 50, but I never had any that failed because they had more than 50. So I never had to set it higher.