Q:about how long would it take to run the exchange update tool on 1600 users?
A: This has several variables, like the processing power of the machine the tool is running on and the number of items in the delta weeks on the affected calendars. Microsoft saw an average of 7-13 mailboxes processed per minute per client. That last part is important. The Exchange tool is single-threaded. To increase through-put, you can run multiple clients against the same server. You would chose non-overlapping lists of mailboxes for each client to process. When MS did theirs, they ran eight clients simultaneously against each server. You don't need to run that many. It's just an option to keep in mind to have multiple clients divide up the workload.